diff --git a/src/helpers/wiring.helper.ts b/src/helpers/wiring.helper.ts
index a301da0..be535d7 100644
--- a/src/helpers/wiring.helper.ts
+++ b/src/helpers/wiring.helper.ts
@@ -591,6 +591,7 @@ export function CreateLibrary<S extends ServiceMap, C extends OptionalModuleConf
   services,
   depends,
   optionalDepends,
+  ...extra
 }: LibraryConfigurationOptions<S, C>): LibraryDefinition<S, C> {
   validateLibrary(libraryName, services);
 
@@ -609,6 +610,8 @@ export function CreateLibrary<S extends ServiceMap, C extends OptionalModuleConf
   }
 
   const library = {
+    // * Merge in stuff which may only exist via declaration merging
+    ...extra,
     [WIRE_PROJECT]: async (
       internal: InternalDefinition,
       WireService: (
diff --git a/src/services/wiring.extension.ts b/src/services/wiring.extension.ts
index fc5db5e..fef4cce 100644
--- a/src/services/wiring.extension.ts
+++ b/src/services/wiring.extension.ts
@@ -217,6 +217,7 @@ export function CreateApplication<S extends ServiceMap, C extends OptionalModule
   libraries = [],
   configuration = {} as C,
   priorityInit = [],
+  ...extra
 }: ApplicationConfigurationOptions<S, C>) {
   let internal: InternalDefinition;
 
@@ -234,6 +235,8 @@ export function CreateApplication<S extends ServiceMap, C extends OptionalModule
 
   const serviceApis = {} as GetApisResult<ServiceMap>;
   const application = {
+    // * Merge in stuff which may only exist via declaration merging
+    ...extra,
     [WIRE_PROJECT]: async (internal: InternalDefinition) => {
       BOILERPLATE(internal)?.configuration?.[LOAD_PROJECT](
         name as keyof LoadedModules,