I have about 6 friends who have taken an unlocked phone to Metro and they refuse to give them a SIM card for it. Metro is the only place I've heard of who refused to do this-you can only use phones from the Metro store unfortunately...
No you can't use your blackberry pearl mobile with metro pcs sim until you unlock it from t-mobile .You can easily unlock your phone using sites like Unlock-Zone.com here they are rendering network unlock code for all mobile models at reliable cost .After purchasing the code visit sites like MobileUnlockGuide.com to get free instruction about how to use that code and unlock your phone .AFter unlocking your phone you can freely use it with any GSM sim ...