The INET framework for the widely used OMNeT++ simulation environment supports discrete event simulation for IP-based networks. During the development of a simulation model for the new IETF transport protocol SCTP (Stream Control Transmission Protocol), INET was extended to also support external interfaces. These interfaces allow to set up hybrid scenarios where simulated nodes communicate with real external IP-based nodes. This paper will first give a short introduction to the OM-NeT++ simulation environment and the INET framework. Then the requirements for the external interfaces will be discussed and some implementation aspects will be described. Hybrid scenarios offer a whole range of potential applications which will also be presented briefly. The performance of this technique is crucial for its applicability. Therefore, several test setups are evaluated to verify the feasibility of this approach.