As PP mentioned, you can do it either way. But unlike the PP, my preference is to wait until I'm there and I actually need the extra day to add it. That's just in case something comes up and I don't actually need the extra day.
ETA: I just remembered. If you booked your package before the June price increase, making a change to your tickets now would mean you would have to not only pay for the extra day, but also pay for the price difference between the prior price and the current price. If you wait until you are at WDW and have used the tickets at least one day, then you will only pay the cost of the 1 extra day.