if these are songs from the Sprint Music Store, it's not going to be possible. Sprint uses a special format called .koz and it's not compatible with anything outside the Music Store app. If however, these are mp3s or wma (windows media audio) files, you should be able to use your USB charging cable and a computer to move the file from a music folder to a ringer folder. see pg 112 from the user guide.
On the Epic and Intercept you simply open the Music Player and find the song. Press and hold the song title until the menu opens up. Then Select as Ringtone. I think it would be the same on your phone.