Matheus, it can't work that way.

Xara exports static layout so the under-text is fixed in position.

If it is important, you could rebuild the NavBar functionality into separate pop-up layers for each sub-menu and have the required under-text "repelled", but you have to cover up that text on the MouseOff layer to achieve the effect.
With suitable transition effects you could make it quite snazzy.

Xara - Sub-Menu Effects.xar

Acorn

P.S. @Xara - another Bug with Text Paragraph Background - it destroys any Mouse-over action!