Certainly double check with a person in reservations, but...
I just spoke with a person yesterday about something similar. My parents might actually go with us in Sept (we have only one room booked for myself, DH and 2 DDs). I asked if we could add a room to our ressie, and if we did, would they also get free dining with us. The woman I spoke with was wonderful and said that if we didn't mind adding the room with ours (basically meaning that we would be liable for the $$, since the whole thing would be in my name), then yes, we could do this, and they would get the free dining as well. Of course, we have to get them the same tickets that we are getting as well (that's the rule).
Thought I would share...hope you can do the same, if you need to!