I would say your best bet is to just spend the money and buy a Sprint phone. Its not completely impossible to use an unlocked phone on CDMA network, however there are no guarantees that certain features like data, text messaging, etc. will work.

I bought a Qwest Samsung M520 Slider on Ebay and want to use it on my Sprint account. I can't seem to get anyone to unlock it or transfer it to Sprint. Is this a lost cause? The previous owner had the service cancelled and the phone is not under contract. Qwest is switching over to Verizon. Sprint uses this same phone in their service. Please help.