Add an extra field to the navigation menu box?

Goal:
I like to use a subtitle in the navigation bar.

The most logic location, in my opinion, would be Appearance > Menu and add a field below navigation label called subtitle. So the end user can easily add and change a subtitle there for the different menu items.

Looking at wp-admin/includes/nav-menu.php, I can’t find an action to hook onto, is this correct? Does anyone have an idea to accomplish this?

have you ticked the ‘Description’ box under ‘Show advanced menu properties’ in the ‘screen options’ tab in the appearance -> menus page?

and have a look at this related post: http://www.kriesi.at/archives/improve-your-wordpress-navigation-menu-output