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.
Sprint is proud to have been named to The Civic 50, an annual survey of America?s most community-minded companies.