First, keep in mind that most carriers I know of (Sprint, T-Mobile, AT&T, etc.) lock their phones so that they can only be used on their original network. A lot of phones are able to be unlocked, but that will most likely cost you. (Example, I unlocked my T-Mobile phone a few years ago to use with Cingular and it cost me roughly $30.) Also, this would most likely void any warranty the phone has.
**Additionally, I am certain that metroPCS would allow you to activate a phone that is not theirs, because I have asked one of the associate at one of the metroPCS store in the past, when I want to get one of the Sprint PCS CDMA phone activate with their service, but they say, we can only activate OUR service with OUR phones, it doesn't matter if it's unlocked or not, even if it's one of the CDMA phones, we still cannot activate a phone that is not ours! Because it is against our company's policy.** After a big NO to my question, the sales people immediately try to sell me one of their metroPCS phones. -_-
I also checked the FAQ on the web site just to make sure, and the answer is very vague. They said "The phones available with metroPCS service were specifically chosen because of their optimal performance on the metroPCS network." This would lead you to believe they don't support other phones.
Lastly, remember that new metroPCS phone purchases include first month of service and activation fee. If you were to activate a phone you already had (metroPCS or otherwise) you'd have to pay these up front. So, in addition to paying to unlock a phone, you'd also be spending roughly another $60-70 just to turn on your service. With their cheapest phone being $99, I'd say it's a much better value and less hassle to purchase a new phone.
Hopefully that helped. If you have more questions, I'd recommend visiting a corporate store, trying to call metroPCS for customer service is practically impossible. Good luck!