Annual Passholders *can* book a room-only reservation and add the DDP. We are FL Resident Seasonal Annual Passholders and just booked a reservation for Dec w/a discounted room rate plus DDP (and have another pkg like that for Sept). But we booked the room as *passholders* rather than using our FL resident discount. As far as I know, there is no resident room-only plus DDP code.
And, as a PP said, you would have to add it in advance, not when you check in.