diff --git a/hugr-core/src/builder/dataflow.rs b/hugr-core/src/builder/dataflow.rs
index cc6b1536fc..a5faa0aef9 100644
--- a/hugr-core/src/builder/dataflow.rs
+++ b/hugr-core/src/builder/dataflow.rs
@@ -259,6 +259,31 @@ impl FunctionBuilder<Hugr> {
     }
 }
 
+impl<B: AsMut<Hugr> + AsRef<Hugr>> FunctionBuilder<B> {
+    /// Initialize a new function definition on the root module of an existing HUGR.
+    ///
+    /// The HUGR's entrypoint will **not** be modified.
+    ///
+    /// # Errors
+    ///
+    /// Error in adding DFG child nodes.
+    pub fn with_hugr(
+        mut hugr: B,
+        name: impl Into<String>,
+        signature: impl Into<PolyFuncType>,
+    ) -> Result<Self, BuildError> {
+        let signature: PolyFuncType = signature.into();
+        let body = signature.body().clone();
+        let op = ops::FuncDefn::new(name, signature);
+
+        let module = hugr.as_ref().module_root();
+        let func = hugr.as_mut().add_node_with_parent(module, op);
+
+        let db = DFGBuilder::create_with_io(hugr, func, body)?;
+        Ok(Self::from_dfg_builder(db))
+    }
+}
+
 impl<B: AsMut<Hugr> + AsRef<Hugr>, T> Container for DFGWrapper<B, T> {
     #[inline]
     fn container_node(&self) -> Node {
diff --git a/hugr-core/src/builder/module.rs b/hugr-core/src/builder/module.rs
index b57d25c144..9e045bc2a4 100644
--- a/hugr-core/src/builder/module.rs
+++ b/hugr-core/src/builder/module.rs
@@ -51,6 +51,12 @@ impl HugrBuilder for ModuleBuilder<Hugr> {
 }
 
 impl<T: AsMut<Hugr> + AsRef<Hugr>> ModuleBuilder<T> {
+    /// Continue building a module from an existing hugr.
+    #[must_use]
+    pub fn with_hugr(hugr: T) -> Self {
+        ModuleBuilder(hugr)
+    }
+
     /// Replace a [`ops::FuncDecl`] with [`ops::FuncDefn`] and return a builder for
     /// the defining graph.
     ///
@@ -229,4 +235,19 @@ mod test {
         assert_matches!(build_result, Ok(_));
         Ok(())
     }
+
+    #[test]
+    fn builder_from_existing() -> Result<(), BuildError> {
+        let hugr = Hugr::new();
+
+        let fn_builder = FunctionBuilder::with_hugr(hugr, "main", Signature::new_endo(vec![]))?;
+        let mut hugr = fn_builder.finish_hugr()?;
+
+        let mut module_builder = ModuleBuilder::with_hugr(&mut hugr);
+        module_builder.declare("other", Signature::new_endo(vec![]).into())?;
+
+        hugr.validate()?;
+
+        Ok(())
+    }
 }