With a 10-day Hopper with the Water Parks & More option, you have 10 regular park admissions and 10 Water Park or DisneyQuest admissions. If you plan one day that you only go to a Water Park and DTD, you'll have plenty of admissions to cover your 11-day stay. I honestly can't imagine the need to add an extra main park admission, you'll be exhausted if you manage to use all you've already got. :shrug:
As has been said, you can't add to or upgrade beyond ten total days on a US-purchased ticket [UK has 14- and 21-day tickets available] without switching to an AP. If you've used any of your WP&M admissions, you would have to upgrade to a PAP. A one-day Hopper ticket with WP&M would cost $186.38
A 10-day Premium MYW ticket costs $358.91 [that's what you have].
An AP [no Water Parks] is $499.49 and a PAP is $637.94.
So to get 11 full days of Hopper + WP&M, you can buy a 10-day plus a 1-day for a total of $545.29 or a PAP for $637.94. Realistically, you wouldn't need to get a 10-day and a 1-day Hopper + WP&M unless you actually plan on going to a Water Park and hop between the main parks every day of your 11-day stay [which would kill me]. So if you absolutely must go to at least one park every day and perhaps not hop every day and miss one day of Water Parks, you could keep your 10-day Premium MYW and get a one-day base ticket to cover that 11th main park day for $438.79.