diff --git a/examples/inject.rs b/examples/inject.rs
new file mode 100644
index 0000000..c03d8d4
--- /dev/null
+++ b/examples/inject.rs
@@ -0,0 +1,42 @@
+extern crate parity_wasm;
+
+use std::env;
+
+use parity_wasm::elements;
+
+pub fn inject_nop(opcodes: &mut elements::Opcodes) {
+    use parity_wasm::elements::Opcode::*;
+    for opcode in opcodes.elements_mut().iter_mut() {
+        match opcode {
+            &mut Block(_, ref mut block) | &mut If(_, ref mut block) => {
+                inject_nop(block)
+            },
+            _ => { }
+        }
+    }
+
+    opcodes.elements_mut().insert(0, Nop);
+}
+
+fn main() {
+    let args = env::args().collect::<Vec<_>>();
+    if args.len() != 3 {
+        println!("Usage: {} input_file.wasm output_file.wasm", args[0]);
+        return;
+    }
+
+    let mut module = parity_wasm::deserialize_file(&args[1]).unwrap();
+
+    for section in module.sections_mut() {
+        match section {
+            &mut elements::Section::Code(ref mut code_section) => {
+                for ref mut func_body in code_section.bodies_mut() {
+                    inject_nop(func_body.code_mut());
+                }
+            },
+            _ => { }
+        }
+    }
+
+    parity_wasm::serialize_to_file(&args[2], module).unwrap();
+}
\ No newline at end of file
diff --git a/src/builder/code.rs b/src/builder/code.rs
index 760c63a..d919f97 100644
--- a/src/builder/code.rs
+++ b/src/builder/code.rs
@@ -179,34 +179,37 @@ impl<F> SignaturesBuilder<F> where F: Invoke<SignatureBindings> {
 
 pub struct FuncBodyBuilder<F=Identity> {
     callback: F,
-    locals: Vec<elements::Local>,
-    opcodes: elements::Opcodes,
+    body: elements::FuncBody,
 }
 
 impl<F> FuncBodyBuilder<F> {
     pub fn with_callback(callback: F) -> Self {
         FuncBodyBuilder {
             callback: callback,
-            locals: Vec::new(),
-            opcodes: elements::Opcodes::empty(),
+            body: elements::FuncBody::new(Vec::new(), elements::Opcodes::empty()),
         }
     }
 }
 
 impl<F> FuncBodyBuilder<F> where F: Invoke<elements::FuncBody> {
 
+    pub fn with_func(mut self, func: elements::FuncBody) -> Self {
+        self.body = func;
+        self
+    }
+
     pub fn with_locals(mut self, locals: Vec<elements::Local>) -> Self {
-        self.locals.extend(locals);
+        self.body.locals_mut().extend(locals);
         self
     }
 
     pub fn with_opcodes(mut self, opcodes: elements::Opcodes) -> Self {
-        self.opcodes = opcodes;
+        *self.body.code_mut() = opcodes;
         self
     }
 
     pub fn build(self) -> F::Result {
-        self.callback.invoke(elements::FuncBody::new(self.locals, self.opcodes))
+        self.callback.invoke(self.body)
     }
 }
 
diff --git a/src/builder/mod.rs b/src/builder/mod.rs
index 034114d..6a2436e 100644
--- a/src/builder/mod.rs
+++ b/src/builder/mod.rs
@@ -6,6 +6,6 @@ mod code;
 mod misc;
 mod import;
 
-pub use self::module::{module, ModuleBuilder};
+pub use self::module::{module, from_module, ModuleBuilder};
 pub use self::code::{signatures, function};
 pub use self::import::import;
\ No newline at end of file
diff --git a/src/builder/module.rs b/src/builder/module.rs
index 4f192bc..0eacbd7 100644
--- a/src/builder/module.rs
+++ b/src/builder/module.rs
@@ -1,5 +1,6 @@
 use super::invoke::{Invoke, Identity};
 use super::code::{self, SignaturesBuilder};
+use super::import;
 use elements;
 
 /// Module builder
@@ -90,6 +91,12 @@ impl<F> ModuleBuilder<F> where F: Invoke<elements::Module> {
         }
     }
 
+    /// Builder from raw module
+    pub fn with_module(mut self, module: elements::Module) -> Self {
+        self.module = module.into();
+        self
+    }
+
     /// Fill module with sections from iterator
     pub fn with_sections<I>(mut self, sections: I) -> Self 
         where I: IntoIterator<Item=elements::Section>
@@ -170,6 +177,17 @@ impl<F> ModuleBuilder<F> where F: Invoke<elements::Module> {
         SignaturesBuilder::with_callback(self)
     }
 
+    /// With inserted import entry
+    pub fn with_import(mut self, entry: elements::ImportEntry) -> Self {
+        self.module.import.entries_mut().push(entry);
+        self
+    }
+
+    /// Import entry builder
+    pub fn import(self) -> import::ImportBuilder<Self> {
+        import::ImportBuilder::with_callback(self)
+    }
+
     /// Build module (final step)
     pub fn build(self) -> F::Result {
         self.callback.invoke(self.module.into())
@@ -196,11 +214,26 @@ impl<F> Invoke<code::SignatureBindings> for ModuleBuilder<F>
     }
 }
 
+impl<F> Invoke<elements::ImportEntry> for ModuleBuilder<F>
+    where F: Invoke<elements::Module> 
+{
+    type Result = Self;
+
+    fn invoke(self, entry: elements::ImportEntry) -> Self::Result {
+        self.with_import(entry)
+    }
+}
+
 /// Start new module builder
 pub fn module() -> ModuleBuilder {
     ModuleBuilder::new()
 }
 
+/// Start builder to extend existing module
+pub fn from_module(module: elements::Module) -> ModuleBuilder {
+    ModuleBuilder::new().with_module(module)
+}
+
 #[cfg(test)]
 mod tests {
 
diff --git a/src/elements/func.rs b/src/elements/func.rs
index f022e72..fcef2ad 100644
--- a/src/elements/func.rs
+++ b/src/elements/func.rs
@@ -79,6 +79,12 @@ impl FuncBody {
     /// Opcode sequence of the function body. Minimal opcode sequence
     /// is just `&[Opcode::End]`
     pub fn code(&self) -> &Opcodes { &self.opcodes }
+
+    /// Locals declared in function body (mutable).
+    pub fn locals_mut(&mut self) -> &mut Vec<Local> { &mut self.locals }
+
+    /// Opcode sequence of the function body (mutable).
+    pub fn code_mut(&mut self) -> &mut Opcodes { &mut self.opcodes }
 }
 
 impl Deserialize for FuncBody {
diff --git a/src/elements/ops.rs b/src/elements/ops.rs
index 68710d1..a96c603 100644
--- a/src/elements/ops.rs
+++ b/src/elements/ops.rs
@@ -10,18 +10,21 @@ use super::{
 pub struct Opcodes(Vec<Opcode>);
 
 impl Opcodes {
-    /// New list of opcodes from vector of opcodes
+    /// New list of opcodes from vector of opcodes.
     pub fn new(elements: Vec<Opcode>) -> Self {
         Opcodes(elements)
     }
 
-    /// Empty expression with only `Opcode::End` opcode
+    /// Empty expression with only `Opcode::End` opcode.
     pub fn empty() -> Self {
         Opcodes(vec![Opcode::End])
     }    
 
-    /// List of individual opcodes
+    /// List of individual opcodes.
     pub fn elements(&self) -> &[Opcode] { &self.0 }
+
+    /// Individual opcodes, mutable.
+    pub fn elements_mut(&mut self) -> &mut Vec<Opcode> { &mut self.0 }
 }
 
 impl Deserialize for Opcodes {
diff --git a/src/elements/section.rs b/src/elements/section.rs
index 947eadc..e9e4614 100644
--- a/src/elements/section.rs
+++ b/src/elements/section.rs
@@ -228,6 +228,11 @@ impl ImportSection {
     pub fn entries(&self) -> &[ImportEntry] {
         &self.0
     }
+
+    /// List of import entries (mutable).
+    pub fn entries_mut(&mut self) -> &mut Vec<ImportEntry> {
+        &mut self.0
+    }    
 }
 
 impl Deserialize for ImportSection {
@@ -903,9 +908,9 @@ mod tests {
               9,             //   function #1 total code size
               1,             //   1 local variable declaration
               1,             //      amount of variables
-              0xff,          //      type of variable (-0x01), negative
+              0x7f,          //      type of variable (7-bit, -0x01), negative
               0x02,          //   block
-                0xff,        //      block return type (-0x01), negative
+                0x7f,        //      block return type (7-bit, -0x01), negative
                 0x23, 0x00,  //      get_global(0)
                 0x0b,        //   block end
             0x0b,            // function end