Hi Jochen,
Converting the equates and structure definitions from the SDK C\C++ header files is a difficult task. As you mentioned, some manual tweakings are unavoidable. Japheth,ToutEnMasm and Donkey created their own include file set. Creating include files for API function declarations is easier. You need to read the MS COFF import library specification to understand the structure of import libraries. Converting a statement like _MessageBoxA@16 to a sequence of PROTO and DWORD is easy. To be honest, my converter assumes everything as DWORDs.