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.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.