I wasn't going to reply since this subject has been beaten to death in many other threads, but someone giving incorrect advice forces me to say something.
No, you cannot just flash your device with a Sprint ROM and start using a Sprint PRL. The ESN will not show up as a Sprint ESN, they won't let you activate it and you'll have a nice PDA with no phone functions.
There are threads that deal with changing the ESN to a legal Sprint ESN, but it is NOT a legal procedure. And no, Sprint won't do it either.
I highly doubt that any Sprint employee told you that you "might find something by researching it" (at least in any official capacity).
Look at these two threads:
http://www.ppcgeeks.com/sprint-xv670...hange+esn.html
http://www.ppcgeeks.com/i-changed-my...hange+esn.html