My understanding (and my experience) is that the miles needed for an award are deducted when you first make the award reservation. While it's generally possible (depending on the carrier/s you're trying to ticket) to extend an
existing hold for a series of 3-day periods that could reach beyond 20 days overall, it sounds like the OP wants to reserve an award he/she doesn't yet have the mileage to ticket. If that's the case, since your account would get docked for the miles up front, the short answer is: no. You need to have the proper amount of miles in your account before you make the reservation; there's no award layaway program.