Unfortunately you can't use anything from the SD card as a ringtone, the phone won't recognize it. What you can do however, is make your own ringtones. Using any mp3 file you have on a computer you can upload it as an attatchment with your favorite email client and send it to your phone as a ringtone. In the email send it to email@example.com, be sure to include a subject and it will arrive on your phone as a MMS. Play the message on your phone and you can click "options" and "save as ringtone".
Use ringtone editors to get the bits of song you want. Google: Myxer, Audacity, Funformobile.com ..just to name a few.