It depends on the hotel program and possibly the property as to whether this is possible.

And it also depends on whether you're buying hotel points at the normal rate, or during a promotion when the rate of points to money paid is temporarily improved, or during a special situation like a points+cash booking (where the cash actually just buys the remaining points needed in at least a couple hotel programs). By the way, I don't think that all hotel programs have such periodic point buying promotions.