You have two seperate reservations, so you would have to add a separate dining plan to each one. You cannot buy one dining plan to cover both reservations.
If your AOA reservation is a room-only you cannot add a dining plan to that unless you change it to a package. Unless you're an annual passholder or eligible for the military discount, that means you have to buy park tickets as part of the package. If your AoA stay is first, you could concievably buy tickets with that package to cover your whole stay (if you need tickets). The dining plan is limited to the number of nights on the reservation, but the tickets are not.
As noted, for the
DVC reservation you will need to get the DVC owner to add the dining plan for you, as only they can deal with Member Services, and you'd need to arrange to get the payment for the dining plan to the member. No ticket purchase is required to add
DDP to a DVC reservation.