No. It's not a simple process. But I've seen somewhere on the interwebz where people made it so a Droid X from Verizon could be used on Boost or Cricket. But it's a very technical process using CDMA workshop and you have a chance of bricking the phone and running into a lot of problems.
But yes, in theory, you should be able to take a phone from Sprint or Verizon and make it work with Boost. But again, it is risky and there's a lot of technical information you have to read over. I don't think any carrier will do this for you. But you can always ask Boost's customer service.