How would someone do this?

[Shadownova]

Take the source code for a desktop (win32) program and convert it to a ppc program?

[AnswerDude]

It depends on what type of source code it is, and question of screen size, interfaces and drivers will have to be addressed. But MS provides good starting point:
http://msdn.microsoft.com/mobility/gettingstarted/windowsmobile/default.aspx
http://msdn.microsoft.com/mobility/thekit/