You can use Ringtone maker app from Microsoft to add music as ringtones, but best way is to paste new ringtones into phone memory folder Ringtones. Than they'll be added automatically to your phone settings ringtones+sounds. Hope it helps.
Well, i guess some mp3 songs are protected.. You can do this: Go to your Pc, then open Local disc(c) open Windows folder, open Media folder. In it you'll find wav music files with names like Alarm, etc. Connect your phone to pc and copy one of above mentioned wav files to phone Ringtones folder. Disconnect phone from Pc, go to settings, Ringtones+sounds and scroll up till the top. You should see that wav file there. Repeat procedure with any mp3 file you like, but i think you need to cut them to 30seconds or so. But your best bet is to download Ringtone Maker from store, its Microsoft app, and it works nicely.