I've been doing this for awhile now, for my ringtones.
If you have a Staples near you, they have a Jabra Bluetooth dongle for 9.99.
Yes, changing the .mp3 extension is as simple as renaming it to .qcp.
You can change the size of your .mp3 file and choose what sections of the song
you want to server as your ringer by going to
http://www.ventones.com/make-free-ringtone.aspxThis will allow you to crop the song to the portion you want; after it's done, save it to your PC,
find the file on your PC, and rename the song from mysong.mp3 to mysong.qcp.
After you rename it on your PC, your PC won't recognize it, but that doesn't matter because you're transferring it to your phone, which will recognize it. Best of all, it's still an actual MP3 file, so you get MP3 quality.