My understanding is that it can be done but I don't have specifics on how to do it.
Hey, marin3r101! Sorry to say that we aren't able to support requests like this here, but I know that some of our users will refer this type of ask out to other sites so hopefully with a little hunting you're able to find what you need.
Check out XDA site for this. They may have an answer to put a firmware on it. Are you using AT&T phone but want to use T-mobile firmware on it using T-mobile service and their sim? If so, then XDA site should help with that one. If your phone is unlocked, then I really do not see much of a difference of just changing it, unless your updating the firmware only, not sure if just adding T-mobile one will get you updates from T-mobile either.
I want to install the who shabang since the radio on AT&T seems to not have all the bands. however i have been unable to do it successfully.