How can I create a side menu from some menu subItems in the same page?

Question

I have a horizontal menu, some of the Items have expandable subItems and all this is built with scripts that I do not have access to, I can only add and remove items through the dashboard and style and interact with the html templates (BigCommerce). I would like to reuse one of this menu Items and it's subItems as an additional side navigation. Is there the possibility to do this with only javascript? When I explore the Menu with firebug the output has a similar scheme to this,

<div class="menu">
 <ul>
  <li class="">Item1</li>
  <li class="hasSubMenu">Item2</li> 
   <ul>
    <li>SubItem1</li>
    <li>SubItem2</li>
    <li>SubItem3</li>
  </ul>
  <li class="">Item1</li>
  <li class="hasSubMenu">Item2</li>
 </ul>
</div>

Thanks for any help.