Sprint will in fact accept foreign ESN's, there are factors involved. Like account status, length of account, how many lines are on the account, etc, etc. It takes persistence, and not accepting the "we don't do that"
I have seen it done, I've charge to get it done. My previous employer is using ten Verizon Razors and 2 BlackBerries on Sprint so it can be done. But like I said earlier it's not easy.

Here's some basic guidelines for adding to Sprint and they will very from Manager to Manager. (Never talk to a CS Rep)

Account needs to be good standing for previous 12 months,
Device must fit into a class of devices they carry,
Age of account should be 2 yrs or older,
The more lines the better,
Be friendly,
check local telecoms porting laws,
check any agreements, terms of service, contracts, "even ones signed with dealers",
Persistence,


Try not to resort to threats unless needed. You can let them know that you need the device for a specific reason, "that none of theirs currently offer" and your not taking "NO" for answer.