Add characters to the press and hold character picker in OS X Lion

In OS X Lion, there is a very useful feature for people like me who need to write in multiple languages, which pops up a small menu of additional characters when you hold down a letter.

For instance, when I hold down the letter E, I get:

This is great when I'm writing in French for example. However right now I need to write something in Czech; however, the Czech letter ě (looks like the 3rd letter in the popup with the accent flipped) does not appear in the popup.

Is there a way to tell OS X to include Czech or other additional characters in those popups?

Note: I do not want to use the Czech keyboard layout; I found switching layouts for each language is somewhat unpractical.