Like PP mentioned though, you are taking a big risk waiting until that point to add days.
Most likely many of the parks will have already been reserved and you will not get the choice that you wanted, or any choice to reserve a park. If you are considering an extra day I would highly recommend adding it now. In our case, for our family of five, it was only around $65 to add the extra day. I would rather spend that now and risk not using that day, than want to have that extra day, try to add it while we are there and be shut out of our desired park.
Dan
We didn't get a ticket for our arrival day bc we just want to swim at the pool and have dinner and also our flight could be delayed. Our arrival day is already sold out, but in case it is raining and we cant swim I would try to add a day. It probably won't be possible. We can always go to Disney Springs. I don't think we will get to our room until 5 pm or later anyway.