public function addMenuItem(\Gems_Menu_SubMenuItem $menuItem, $label = null)
    {
        $key = $this->_getKey($menuItem->get('controller'), $menuItem->get('action'));

        if ($label) {
            $this->altLabels[$key] = $label;
        }

        $this->offsetSet($key, $menuItem);
        return $this;
    }