The OP is wanting to add a day to an existing reservation.
The OTU points can't be used to substitute Home Resort points used for a reservation during the 11 month priority since MS won't allow the purchase of OTU points if there are still current points in the contract available. If there is availability at 7 months a member could cancel up to 24 points worth of nights in the original reservation, use those points for another reservation (or bank them) and then call MS to rebook those nights using OTU points - but they would risk the loss of the nights cancelled since they could disappear before being able to cancel, bank and then call MS to reserve again.