Firstly, you’d have to re-do the sprite with all of the sub-navigation and go from there.
You might also want to look at this:https://css-tricks.com/css-sprites-workflow/
I know sprites are the ‘in’ think right now but they are incredibly inflexible when it comes to scaling or updating your nav.
From what I can see, the only thing the sprite is doing now is adding a text-shadow which is incredibly simple. Obviously, that’s an issue for older browsers but a simple fallback would suffice there.