Yes you can, and you can apply the AP room discount to the same room with the dining plan. The only downside is that in order to add the dining plan, you must convert your reservation to an annual passholder room/dining PACKAGE. This means your reservation will then be subject to the same deposit, change, payment and cancellation policies as all other package reservations. These policies are very different from the policies for room-only reservations, so you should ask for the details if you are not already aware of them.