The maximum number of days that a Magic Your Way ticket can hold is 10 days, so unfortunately the OP is up against that limit with no way to add more days.
Your options pretty much are:
-Skip a day somewhere along your trip and do not go to a park at all. Cost: free.
-Buy a 1-day park ticket for about $95
-Upgrade your 10-day hopper to an Annual Pass for about $212.
-Go to a water park one of the days for about $55, or if you'll go to more than 2 water parks in your trip, the Water Parks & More option is $61 more. (If you don't already have the tickets and they're just part of your package, you might be able to get the discounted price when you get both Hopper and WP&M together, which should only be about $25 more than what you've got now)
-Rob