My question, can I modify the user32.spec and the winuser.h to add this function with?
Yes, but bear in mind you will need to "implement" it, just an empty function with a FIXME("stub\n"); will do.
And an other question. After modifying the user32.spec to rebuild, do I just type make?
Yes. Well, make install, if you're running from a copy of wine outside the tree. Same as any other program.