// Add a Beaver Builder Template to the bottom of the Beaver Builder Theme vertical menu
add_filter( 'wp_nav_menu_items', 'your_custom_menu_item', 10, 2 );
function your_custom_menu_item ( $items, $args ) {
    // if the menu is in fact our header menu
    if ($args->theme_location == 'header') {
        // get the content of our Beaver Builder Template
    	$bb_content = do_shortcode('[fl_builder_insert_layout slug="vertical-menu-content-bottom"]');
    	// append the content of our Beaver Builder Template to the end of our menu
        $items = $bb_content . $items;
    }
    // return our menu + Beaver Builder content ready for output
    return $items;
}