Through the AP website, I was able to book a room reservation with the DDP, without it being a AP discounted rate.
I was told by a phone CM that this isn't possible, however I was able to do it online...
If you already have a booked room-only reservation, and want to add the DDP, you will have to cancel the room-only and rebook as a package. You cannot add the DDP to an existing room-only reservation.
ADDED TO SAY: Since you know for sure you want the DDP, I would suggest booking it now as the Room/DDP package. Once the AP rates are put out, you should be able to apply the rate to your room, as long as the category/hotel you want is available for the discounted rate.