Great question chrisathull72 and thanks for post!
You can absolutely activate service at a T-Mobile store. Just a heads up, you'll want to request an unlock for your Metro phone before putting a T-Mobile SIM in it. After that, you should be good to go my friend! Let us know if you have more questions.
T-Mobile doesn't really activate a particular phone the way that Sprint does. They provide a SIM that can be used in any compatible phone. They also sell phones that are compatible.
What you are really asking is if a T-Mobile SIM will work in a MetroPCS phone, assuming that the phone is SIM-locked to MetroPCS. I'm not really sure but I believe that the answer would be yes. Have you investigated getting MetroPCS to unlock your phone?