Hi,
hmm.. it won't be easy - because it request some code customization in layout.
So I have other solution for you.
1) Disable default menu (it's possible from template options)
2) In TopBar insert another menu module, but more advanced than default one. On JED you will find at least five, for example: Maxi Menu CK, RokNavMenu etc. MORE here:
http://extensions.joomla.org/extensions ... nu-systems