Model simulation is a credible approach for model validation, complementary to others such as formal verification and testing. For UML 2.x, model simulations are available for state machines and communication diagrams; alternative finer-grained simulations, e.g., as are supported for Executable UML, are not available without significant effort (e.g., via profiles or model transformations). We present a flexible, plug-in based approach to enhance UML model simulation. We show how an existing simulation tool applicable to UML behavioural models can be extended to support external action language processors. The presented approach paves the way to enrich existing UML-based simulation tools with the ability to simulate external action languages.