I'm not that familiar with Airvoice, but could you switch to a paygo plan and transfer your balance? It seems to me that you probably should have used paygo from the start, given your current situation with the monthly plan.
Their PAYG rates are
considerably higher on the voice and SMS rates than the $10 plan (10¢/min or SMS), and I can guarantee that switching to it with the current usage pattern on the $10 plan is only going to cost more than what's currently being paid... not that you can carry balance on plan switches, IIRC.
There is no hard limit on rollover balance or set expiration date, but I wouldn't expect them to let it snowball too big. Just start using your phone a bit more liberally if you want, or if you get good T-Mobile reception, just consider switching to P'tel instead so you can get close to the same rates and only have to fork out about $5/month for it.