Dial *99 on your metro phone and add money to your phone, just like you would if you were buying ringtones and games.
You can't I was actually wondering about the same thing so I called the customer service and they said that the service metropcs offers is not compatible so you can't make or receive collect calls...which sucks! And the GTL co. Is a rip off!
Call metro PCS customer service. Or go online and get customer service number. Call and you should be able to add it to your plan. Or visit a corporate store.