No it's impossible.
Because both are different network provider.Once unlocked your CDMA based phone.you will be to CDMA network only.You can unlock your iphone 4s using remote unlocking method.It's the best way to unlock your mobile phone.You can get unlocking service from any one of the online unlocking service provider and you can get free unlock instructions from Mobileunlockguide.com It's very useful do your unlocking.