Add on points are points in a new contract that you buy to add onto your membership. So points that you own.
OTUP are points you can buy when in the process of booking a reservation at 7 months or closer to checkin and don’t quite have enough of your own points available. You have to call MS to buy them, because essentially they’re
DVC’s points that are being transferred to you. And in your scenario, yes, you could buy OTUP, a maximum of 24 per UY at $20 each, if you banked your own points and then planned a quick trip.