If it is a T-Mobile phone, you should be able to use it. Just get a prepaid SIM and load it up with minutes. My daughter has used her BB on postpaid T-Mobile and prepaid T-Mobile with no problems.
If the phone is still active on T-Mobile postpaid, you can just call customer service and they will switch it to prepaid and you won't even have to get a new SIM card. If it isn't active now, and you want a new phone number, I think getting a new SIM would be the easiest thing to do.