I don't think you can do that.
It might happen in the future though as there is potential revenue in it for AirBnB: it implies holding on to your money (instead of paying it our right away) and then applying it to a future stay. That's very attractive to AirBnB for so many reasons: no money transfer costs, cash flow / interest benefit of holding on to your payout, and then finally it means making money on your booking when you become a guest who books a stay.
So it'll probably happen, but I don't think you can do that yet.