Custom field to wp_nav_menu object

hello,
thank you in advance for your help! i’ve post this question under themes, but perhaps this maybe more specific. i’m a noob to wp and i’ve been trying to find a way to add a setting/property/custom meta field to a wp_nav_menu object to determine if it’s global or not. i’ve seen the other ways in which to do this, but i want something a bit more elegant to flag a site’s navigation as global in a multisite set up.

thanks,
~rob