Go into your Sprint store and ask them. They should have a prepay plan. Keeping your number is no problem. Whether they'll accept your phone might be - it's up to them.
And if they won't, you'd have to pay to have the phone modified to be used on one of their MVNOs - if they'd accept it that way.
Before you get into anything complex, just go into the store and ask. They may just switch you over in 5 minutes.