Please check out this similar question with two directions you could use to solve your problem:
Adding html elements to wp nav menu
I would try using either the wp_get_nav_menu_items() function or try using the ‘wp_nav_menu_items’ filter.
An easier solution (If you do not want to do much coding) would be to use The Uber Menu Plugin. wpmegamenu.com/ I have used this on projects before and have been quite pleased.