I want to start an Tmobile plan with an existing AT&T Phone?

I want to start a flex plan with my faves services. But I don't want any of the phones tmobile has because I already have a $300 windows mobile 6 GSM unlocked phone I was using with my AT&T service (Samsung BlackJack II to be exact). Is there anyway I can get the service and not have to buy one of their phones? Also to use email and internet is there a service I can, or do I have to pay for it as extra?