Assuming your phone is Sim-Unlocked..... According to a T-Mobile Rep:
"Since the phone isn't a T-Mobile branded device and the updates are pushed by the Carrier, we don't have any way to issue an update."
I think the only way around this would be to find someone with an AT&T Sim Card... insert it into your phone and check for any needed updates through AT&T. When it's finished just reinsert your original Sim Card and delete the AT&T bloatware apps.
UPDATE....
I am not 100% Certain... but have you tried using Samsung Sync or Samsung Kies to update your phone? I recall Samsung Sync asking if I wanted to update one of my devices, which I didn't because it's rooted. If your phone is unlocked it might or should work..