There is a simple way to add .c and .h files into your build. Just drop your file into the /gen folder of the project. It will automatically be copied into the /src folder during model translation. It is also possible to modify a generated file by copying a file from /src to /gen and editing it. The files in /gen are copied _on top of_ files in /src after all code is generated.
This process is also described in the BridgePoint Model Compiler Help topic in the tool.