Jack's approach is the sanest for a strange request.

Do your NavBars have submenus?
If so, your submenus will fall off the bottom of the page and Xara will have calculated the excess drop needed and add more Pasteboard to compensate.
This would probably upset the aesthetic of your design.

If your navigation does not use submenus then you can create a line of buttons instead and repeat on all pages.

You cannot combine a Live Copy with a Repeat on all pages as, I believe, Xara has excluded this to avoid a coding clash as the techniques being used are the same.

for interest, Xara's button designs are invariably grouped objects so the text, while editable in Xara, ends up as a graphic part of the button. I prefer using CSS3 or my own soft-group designs for buttons and CSS3 NavBars as they are cleaner.

Do come back with how you solved your design.

All the best,
Acorn