I've been looking to do the same actually, except I am trying to flash a friend's Verizon Pearl to her metro account, and unfortunately for you, Sprint will not activate any phones with an unknown ESN (pretty much any phone not sprint branded). Here is the info i've gathered if you are interested anyway:
1. obtain your MSL code
2. Make sure verizon want to activate unknown ESN (sprint ESN) in their network
3. Google for a software call qpst, install it in your PC
4. Uninstall Desktop Manager, and all blackberry software from your PC
5. download this Custom Action Information
6. Restart your computer, then go to your Device Manager, and check under Ports (COM & LPT) there should be 2 serial Port for RIM (e.g: COM3 and COM4), now you need to remember the highest number COM port for your blackberry.
7. Turn off your wireless connection on BB, connect it to your PC via USB, then run Sprint Mobile Broadband
8. Run QPST Configuration, click on Ports tab and add new port (add your blackberry port, the highest number port)
9. Then click Start Clients - Service Programming - Select your phone, for phone model select SURF6300-BB
10. Enter your MSL code, click on Read from Phone
11. Download verizon prl from here Corolada PRL Page then save it to your computer
12. back in qpst, click on Roam tab, under preferred roaming, load verizon prl
13. click on settings tab, change service programming to 000000 (6 zeros)
14. click write to phone
15. phone will reboot.
16 disconnect phone, and call *228 to activate your phone in verizon network
Should be the same from verizon to sprint, well, it's basically the same method, verizon MSL code is 000000 and download Sprint PRL from the same site
but you won't be able to activate it under sprint network, because sprint wont activate unknown ESN on their network.
Here's a video:
Code
http://www.youtube.com/watch?v=us-S-wU4hxg