Hi Necole,
It is not possible to add an 11th day onto your 10 day Magic Your Way park tickets.
The reason why park tickets are only available up to a maximum of 10 days is because after this point, it is most likely to be more cost effective for guests to purchase an Annual Pass.
As Annual Passes are valid for an entire year, if you are planning to return to Walt Disney World at any other time in the 12 months following your trip, it would almost be like getting your park tickets for free. As an additional bonus, Annual Passholders also get offered great discounts
on resorts, dining and special events.
It is possible to upgrade your 10 day Magic Your Way park tickets to Annual Passes and this may be something which is worth considering. You can find out more about Annual Passes on the Walt Disney World website
HERE
Wishing you a magical trip!