diff --git a/src/flow_service/mod.rs b/src/flow_service/mod.rs
index bc20af1..d442e4d 100644
--- a/src/flow_service/mod.rs
+++ b/src/flow_service/mod.rs
@@ -5,7 +5,7 @@ use crate::{
 use tonic::{Extensions, Request, transport::Channel};
 use tucana::{
     aquila::{ModuleUpdateRequest, module_service_client::ModuleServiceClient},
-    shared::Module,
+    shared::{Module, ModuleDefinition},
 };
 
 pub mod auth;
@@ -18,6 +18,11 @@ pub struct FlowUpdateService {
     definition_source: Option<String>,
 }
 
+pub struct ModuleDefinitionAppendix {
+    pub module_identifier: String,
+    pub definitions: Vec<ModuleDefinition>,
+}
+
 impl FlowUpdateService {
     /// Create a new FlowUpdateService instance from an Aquila URL and a definition path.
     ///
@@ -43,6 +48,11 @@ impl FlowUpdateService {
         }
     }
 
+    pub fn with_appendix(mut self, appendices: Vec<ModuleDefinitionAppendix>) -> Self {
+        append_definitions_to_matching_modules(&mut self.modules, appendices);
+        self
+    }
+
     pub fn with_definition_source(mut self, source: String) -> Self {
         self.definition_source = Some(source);
         self
@@ -97,6 +107,19 @@ impl FlowUpdateService {
     }
 }
 
+fn append_definitions_to_matching_modules(
+    modules: &mut [Module],
+    appendices: Vec<ModuleDefinitionAppendix>,
+) {
+    for appendix in appendices {
+        for module in modules.iter_mut() {
+            if module.identifier == appendix.module_identifier {
+                module.definitions.extend(appendix.definitions.clone());
+            }
+        }
+    }
+}
+
 fn apply_definition_source_to_module(mut module: Module, source: String) -> Module {
     for data_type in &mut module.definition_data_types {
         data_type.definition_source = source.clone();
@@ -116,3 +139,80 @@ fn apply_definition_source_to_module(mut module: Module, source: String) -> Modu
 
     module
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn module(identifier: &str) -> Module {
+        Module {
+            identifier: identifier.to_string(),
+            name: Vec::new(),
+            description: Vec::new(),
+            documentation: String::new(),
+            author: String::new(),
+            icon: String::new(),
+            version: String::new(),
+            flow_types: Vec::new(),
+            runtime_flow_types: Vec::new(),
+            function_definitions: Vec::new(),
+            runtime_function_definitions: Vec::new(),
+            definition_data_types: Vec::new(),
+            configurations: Vec::new(),
+            definitions: Vec::new(),
+        }
+    }
+
+    fn definition(identifier: &str) -> ModuleDefinition {
+        ModuleDefinition {
+            flow_type_identifier: vec![identifier.to_string()],
+            value: None,
+        }
+    }
+
+    #[test]
+    fn appends_appendix_definitions_to_all_modules_with_same_identifier() {
+        let mut modules = vec![module("shared"), module("other"), module("shared")];
+        let appendices = vec![ModuleDefinitionAppendix {
+            module_identifier: "shared".to_string(),
+            definitions: vec![definition("flow")],
+        }];
+
+        append_definitions_to_matching_modules(&mut modules, appendices);
+
+        assert_eq!(modules[0].definitions.len(), 1);
+        assert_eq!(modules[1].definitions.len(), 0);
+        assert_eq!(modules[2].definitions.len(), 1);
+    }
+
+    #[test]
+    fn appends_multiple_matching_appendices() {
+        let mut modules = vec![module("shared")];
+        let appendices = vec![
+            ModuleDefinitionAppendix {
+                module_identifier: "shared".to_string(),
+                definitions: vec![definition("flow-a")],
+            },
+            ModuleDefinitionAppendix {
+                module_identifier: "shared".to_string(),
+                definitions: vec![definition("flow-b")],
+            },
+            ModuleDefinitionAppendix {
+                module_identifier: "other".to_string(),
+                definitions: vec![definition("flow-c")],
+            },
+        ];
+
+        append_definitions_to_matching_modules(&mut modules, appendices);
+
+        assert_eq!(modules[0].definitions.len(), 2);
+        assert_eq!(
+            modules[0].definitions[0].flow_type_identifier,
+            vec!["flow-a".to_string()]
+        );
+        assert_eq!(
+            modules[0].definitions[1].flow_type_identifier,
+            vec!["flow-b".to_string()]
+        );
+    }
+}