Hallo!
The easy way is: Simply write your pix_blabla.cpp file in the folder src/Pixex and the run the configure script again or add it to src/PixesMake.source.
Or you can make a GEM-external and the link it to Gem.pd_linux (or whatever) - I can send you an example makefile if you need (but only for linux ..)
LG Georg