I admit get Disney discounts and pricing can be confusing and unpredictable (and hard to verify because they never give you an itemized breakdown), BUT I always book a package and have had no troubles changing the #of ticket days for only the incremental ticket price (I.e. About $10-11 per person once you have a base ticket of 5 days or so).
Do you have some kind of special offer already? Free dining, room only, discounted tickets? I don't know why it should matter, but again, sometimes their pricing is inexplicable. I have a MYW package for Oct and have changed ticket days several times already and even had a room only discount applied - each time the only price change was isolated to the ticket price or room night price, no other wonky changes took place just because I have a package reservation. Seems worth it to make another call. Jot down your current price, ask them to recalculate with the added ticket day, and see how it stacks up.