Hi Cediland,
Creating/editing a GPI-DLL requires a Visual C 6.0 compiler and knowledge of creating DLLs from C. You can find DIAdem's GPI-DLL Wizard and example code at:
http://digital.ni.com/softlib.nsf/websearch/D605AA96CF81760C86256C7600742EC5?opendocument&node=132060_US
If you wish, I can see if NI originally wrote the GPI-DLL, and if we still have the C source code, I might be able to send it to you as a starting point.
Regards,
Brad Turpin
NI
brad.turpin@ni.com