As I don't have your specific Navbar handy, it is possible to Ungroup the elements by double-clicking it and then clicking the Ungroup to edit graphics button.
Each button will be grouped so you need to ungroup each in the same fashion and then manipulate the text. There are a lot of steps and nuances to getting this right so I wouldn't even try.

Your example isn't really a Xara-style NavBar, it is a collection of text links on a graphic. It doesn't have sub-menus.

In this case, all you need to is create a number of text boxes of the correct depth. Add text.
Clone and colour the text to the rollover colour and place on the MouseOver layer. Soft-group with the original text and add the link.
Repeat for all links align and place on top of your link bar graphic.

acorn