Add a "dropdown" class to the sub-nav ul element for Foundation off canvas nav menus. For use with the wp_nav_menu() Walker argument.

class_foundation_offcanvas_nav_menu.php

/*
    Add a "dropdown" class to the sub-nav ul element for Foundation off canvas nav menus
*/
class Foundation_Offcanvas_Walker_Nav_Menu extends Walker_Nav_Menu {
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul class=\"right-submenu\">\n";
        $output .= '<li class="back"><a href="#">Back</a></li>';
    }
}