The answer is yes! You can bring a used cell phone to a plan quite easily. If it's a GSM phone like T-Mobile or Cingular you simply pop in an appropriate Chip (SIM Card) and assuming the phone is not LOCKED to another network - you will have no problem. If it's CDMA you will need to register the new phone with the network and ask them for a phone number for it.
For more information on Cell Phone trends see: http://www.mphone.net
Be careful doing this though...whether the company provides the phone or not, it WILL add a contract when the new number is created.
It is not recommended but can be done. If you want to add an additional line, the best thing to do is get a new phone. It is your only opportunity get get a free phone when your 2 year contract starts. If you don't take advantage of this, you will be out of luck. Also, you can get a nice phone free on any cell phone plan, especially an unlimited cell phone plan.