There is a need for integration of external models in high-level system design flows. We introduce a set of partial refinement operations to implement models of heterogeneous embedded systems. The models are in form of process networks where each process belongs to a single model of computation. A semi-formal design flow has been introduced based on these operations to incrementally refine system specifications to their implementation. Wrapper processes, which allow co-simulation of a system model in the framework with external models and implementations are used to keep the intermediate system models after each refinement step verifiable. Additionally, this design flow has the advantage of integrating legacy code and IP cores. Using a simple example as the case study, we have shown how we can apply this design methodology to a simple system.