C++ developer to produce SDK of existing C# Win32 application for Mac and PC

We have an application that is proof-of-concept. We now need to take that application and its functionality and produce a C++ SDK that would allow a programmer to integrate our functionality into theirs. The end product should consist of C++ SDK, all required files, headers and libraries, and high quality documentation. SDK should be cross-platform, so ideally usable on both PC and Mac. The application gathers data on a users PC, posts data in an XML file to a LAMP server and returns the response in a function.