We've been down a lot of routes for this one! Virgin will only let you add it to an existing booking if you are staying on Disney property for the entire length of your hols, and not with a wedding package. Seems very petty to us, and another reason this will be our last trip with Virgin.
We went direct to Disney who say they 'legally' cannot add it to a booking made thru' an operator (like Virgin).
So, our decision was made for us, but we do have a car for the 9 nights we'll be in AKL so Disney will lose out on our money as we'll be going off site like previous times.