In simple terms no it is not possiable. But you can use iringer to convert music tracks to iphone ringtones. Just select the track you want as a ringtone and the program does the rest. I surgest you make a copy of the track before you convert it with the program. As it messed up a track I brought form itunes.
Edit the length of the track in itunes to no more than 40 seconds (or 30 seconds I cont remember which) Then change the file extension to M4r. Then copy to itunes and it will file it in your ringtones folder for sync to your iphone.