Yes, when booking a package everyone in the room must have the same thing (tickets and dining plan). But adding an extra person after check-in is a whole different ballgame.
Disney cannot require them to have the same dining plan, because they won't add it on that late. And my understanding is that Disney will not stop you from adding a person who is not on the original booking to a room as long as it does not exceed occupancy limits.
As I said, I'm not 100% sure, and would suggest getting further confirmation before planning on it, but honestly I wouldn't call...Phone CMs can be the worst. For Disney contact that has the best chance of being correct, I suggest the e-mail option. That way, you have a record of it in case you run into problems and I also think there's less reason for a CM who doesn't have a clue what the truth is to make something up just to give you an answer when it's an e-mail that should get a response within a few days vs. a customer who gets increasingly irritated and impatient when waiting on the phone.
Edited to add: I bet the resort board would be a great place to clarify this policy if you'd prefer a DIS answer to Disney!