I would think you could. The old America's Choice plan for 450 min for $39.99 is now the NationWide Individual Basic plan. It has the same minutes for the same price. I would think you would be able to add the BB data plan to your old plan.
Even if you had to switch over to a Nationwide vs the Amer. Choice, the minutes/prices are comparable; the difference is in the way data usage is billed, and with the BB, you have a data plan so it would be a non-issue anyway.
Suzy is right. The only difference between AC and NW plans is how data is billed for basic feature phones. Since you're getting an advanced device that requires a data plan anyway, it really negates the whole difference. But yes, if you're so inclined you can actually keep the America's Choice plan and add the Blackberry data plan with it.