Can you turn any t-mobile phone into a prepaid phone?

I know that there is this like prepaid avtvation card and i was wondering if i could use it on any t-mobile cell phone. For example i want to get a new sidekick and i cant get one in lease it is prepaid and i looked on the internet and found some prepaid card thing from t-mobile so can i put it on the phone somehow and make it prepaid??