A few questions to clarify your specific situation will help us to better help you...
Do you want to swap phones and use the BlackBerry instead of your current phone? I don't think you can add a line to a pre-paid account to use both - it would have to be two separate accounts.
What model BB is it? And is it Verizon branded? It may or may not be usable on a pre-paid account.
Finally, what is your current phone and account set up? You may need to get a different pre-paid plan to use the BB, if it is usable on Verizon.