DW is using some points and go to Orlando next year for a conference. I cannot go for the whole stay but might be able to join her there. Is that possible? If so, how do I tell MS?
Just have your name on the reservation and you'll be fine - unless the DDP is involved where you will need to purchase it for all days of the reservation for everyone staying in the room.
With your name on the reservation you will be able to check-in and get a room key when you arrive - or she can get a key for you and just meet someplace when you arrive. You can even set up Magical Express for your arrival and departure.