I'm going to WDW May 3rd-10th and was able to use my AP to get both a discount on the room, then later added the DxDDP, keeping my room discount. Is that what you're asking? Adding the
DDP turns it from a room only ressie to a pkg. Meaning that it is now subject to all the restrictions of a pkg (must be paid in full by 45 days out and must pay a penalty if any changes are made). The CM also tried to add insurance to the pkg and I had to tell him to take it off.
If you are asking about using an AP for a room discount and then adding
free dining. That is not allowed. I'm also going back in Sept for
free dining and had to pay rack rate for the room in order to get the dining for free. However, if AP room discounts happen to be a better deal for that time, I could always cancel my current pkg, take advantage of the AP room discount, then pay for the DDP. With three people in the room, I doubt the AP room discount could beat free dining.