Ok so i was playing round with this my self and if you navigate to /System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/Ringtones/
(cmd, shift g in finder) and paste a ringtone (convert to AAC using itines and change extension to .m4r) and paste the ringtone. It wouldn't show up in feature setting but if you pick one you done like, delete it and rename your ringtone with the same name (I used Apex) it should work perfectly.
Hope this helps someone
dang, I'm having the same issue as OP, but I feel this is a little over my head as far as converting and changing extensions....