Traditional building simulation tools have achieved considerable success in the past. They provide the essential foundation for modeling highly sophisticated tasks. Nevertheless, new challenges and current progress in the energy domain require rapid prototyping capabilities for just-in-time model-based investigation. Supporting these requirements is one of the many advantages of employing modern universal modeling languages. This work addresses the integration of the modern modeling language Modelica with the traditional simulation tool TRNSYS. Using the modern standard functional mock-up interface for tools interoperability, a straightforward way for Modelica-enabled rapid prototyping within TRNSYS is presented.