Add menu item in hook form submit
Value of plid is the menu link id of the item you want your new item to be a child of (=parent id).

To create your link, simply use something like:

$item = array(
  'link_path' =&gt; 'node/' . $node-&gt;nid,
  'link_title' =&gt; $node-&gt;title,
  'plid' =&gt; 426,
);
menu_link_save($item);