The classes are not added by the Twenty Ten theme, but by the common custom navigation menu code, in _wp_menu_item_classes_by_context()
, which is called from wp_nav_menu()
.
If you want to add extra classes you can do that by either hooking into wp_nav_menu_objects
, called once with the whole menu tree, or into nav_menu_css_class
, called when rendering each individual item.