I would check this thread out first... It will help you unlock your phone, and if you are so inclined, creating your own custom ROM to put your xv6800 on Sprint...
http://forum.ppcgeeks.com/showthread.php?t=33857
Then you would want to upload a Sprint ROM onto your phone.. After that, you would have to call Sprint and have them activate the phone... Obviously, I glossed over a lot of details, but that is the basic way to go...