Hey there! That's a great question! As far as I can tell, the only ways to add a data pass are through my.t-mobile.com or by calling 611. This is for postpaid customers. Prepaid can add a pass by calling 611 or 1-877-746-0909. You will be required to provide payment information at the time of purchase.
Sorry! It doesn't look like they can be added at the retail level.
So it is only possible to buy a T-Mobile data pass if you happen to have a US credit card? Tourists cannot buy a T-Mobile data pass?
Unfortunately, it would seem so. I dug around a bit and wasn't able to find any information that they payment system would accept a foreign card. If someone knows or has experienced differently, I hope they'll post here!
EDIT: I was able to find that the T-Mobile payment system only can accept:
- ATM/debit cards backed by NYCE, Pulse, Star, and Accel networks
- Credit cards backed by Visa, MasterCard, American Express, and Discover
- Prepaid credit or gift cards with a major card logo