If by "open" you mean unlock then yes, there are third party service that can unlock them.

http://www.gsmlocker.com/blackberry-unlock.htm
http://www.unlockitnow.com/Blackberry/

Keep in mind though that with a prepaid sim none of the data services on the device will work, it'll only be a phone with the ability to send/receive text messages. No email, and no browser. You'd be better off getting a $20 Nokia phone instead.