You can flash Sprint phone to work on Boost or Virgin Mobile but bot the reverse.
Why? To activate a phone on Sprint, the ESN needs to be in their database. Prepaid phones ESN are not registered in Sprint database.
If you find someone who can do that for you, s/he will loose her/his job.
Since it's on the same CDMA spectrum, you can't just have the phone unlocked and have the kernel or the OS flashed to work on Sprint?? Is it possible to take my Epic 4g Touch to boost from Sprint since Boost now offers the same phone?