While you cannot technically "add" a ticket to an existing order in the sense of merging them into one line item, you can upgrade or modify your existing Disney tickets after purchase, provided they have not yet expired or been fully used. If you have a 3-day ticket and decide you want to stay for a 4th day, or if you want to add the "Park Hopper" or "Water Park and Sports" option, you can do so through the My Disney Experience app or at any ticket booth. You will simply pay the difference between the price of the ticket you currently have and the current price of the ticket you want. However, you must perform this upgrade before the last day of your current ticket is used. It is important to note that you cannot "downgrade" a ticket or remove options for a refund once the purchase is finalized. If you simply want to buy a completely new ticket for a friend or another trip, you can do that at any time, but it will be treated as a separate transaction. Always check the "Park Reservation" availability if you are adding days, as a ticket does not guarantee entry without a valid reservation for that specific date.