You can't modify a reservation on-line, you can only cancel all nights or add (and pay for) a dining plan.
If you try to add a day on-line, you would have to book a one night reservation. By doing this, you will end up with multiple consecutive reservations. You would then need to contact MS and have them consolidated into a single reservation to prevent you from needed to checkout and then back in between each segment.
You are much better off calling MS and have them modify the check-out date to extend your reservation (subject to availability).