Basically an M4P file is an MP3 file that Apple laid a protective encryption on (DRM) just like Napster did with their stuff.

The other way of doing it is to leave the DRM protected file in itunes and then just download the mp3 from the millions of free mp3 download sites... I have used a few in the past like http://www.mp3raid.com etc which are pretty good. Saves paying Apple for something else