Ask at a Sprint store, but I dount that you can do it.
There's no SIM in the phone, so it can't be SIM-locked. (That's all "locking" does - prevents you from using a SIM card from a different carrier - in phones that use SIM cards.)
Flashing it still leaves it a Boost Mobile phone. You could probably rewrite the firmware and radio to make it work like a Sprint phone, but that's one of those "if you have to ask, you can't do it" things. You'd have to be both a computer programmer (and a pretty good one) and a cellphone engineer. (After a 40 year career of programming, I still couldn't just sit down and do it.)