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You can contact T-Mobile via phone, facebook or twitter to have that info added to your account.
SSN was asked for credit check with me, but of course I'm naturalized, have a SSN, Driver's License and a credit history.
You must migrate to a postpaid account. At the moment your account is considered no more than a prepaid. You must go to a retail location/call customer care and run your credit and then your number will be migrated over to postpaid to the T-Mobile ONE rate plan. Depending on your credit class rating you may possibly have to put down a deposit. But this will be determined by the credit check, of course.
Basically you cannot add a social security number to a prepaid account.