Add submenu in menupopup mozilla extension developmet?

Here I am working with firefox extension development. I have using XUL Overlays to add menu popup item in my extension.

          <menupopup id="menu_ToolsPopup"> 
             <menuitem label="HP Scheduler Options..."
                       insertafter="devToolsSeparator" 
                       oncommand="openPreferences('hpschedPane');" />
           </menupopup> 

Here I need to add s开发者_开发问答ub menu with menu item.

<menupopup id="menu_ToolsPopup"> 
  <menuitem label="HP Scheduler Options..."
    insertafter="devToolsSeparator" 
    oncommand="openPreferences('hpschedPane');" />
  <menu id="..." label="My submenu!">
    <menupopup>
      <menuitem label="My menu item in the submenu!" oncommand="..." />
    </menupopup>
  </menu>
</menupopup>