Mega menu could be created in Static Content section using Visual Composer shortcodes.
It is possible to edit our demo menu or create your own. The process is same as when creating pages.
To include created mega menu from Static Content in your main menu go Appearance > Customize > Menus. Then click Screen options and enable Static content. It will appear in left sidebar.
After that you will be able to add created “static content” menu in your main menu