Have you searched for this at all?
On WP8 it is possible to put a full-on mp3 as a ringtone. Connect the phone to the PC, browse the phone and you'll see a Ringtones folder. Drop and drag the file you want to be a ringtone into that folder and then when you go to the ringtones+sounds section of the settings it'll be at the top of the list.
Honestly, that's just pathetic, unable to choose a song directly from the Music+Video library. 10yr old Symbian phones could do that, each and every Android phone can do that. I'm not sure about iOS, but I presume even they can.