A CDMA/TDMA wireless handset has a unique electronic serial number, and those ESNs are typically siezed by the respective cellular companies. Basically it is very difficult to move a CDMA/TDMA phone between carriers.
All nokias of that model have an option for prepaid security, it's part of the firmware, but unless the service can be activated as prepaid by the phone's home provider, it will not happen. The prepaid networks use differant station IDs and everything, so the phone would need to be reconfigured.
If the phone is GSM and has a SIM ( Subscriber Information Module ) card, you can just purchase a new prepaid SIM from any GSM provider and convert that phone to prepaid
note that by US law, any cellular handset, active nor not, will be able to call 911
