Add a specific category at a specific place to the menu that uses wp_list_pages

I have a main menu where I display pages. All the pages have associated sub-pages, however in the menu some of them are displayed by themselves without sub-pages and some are displayed with a submenu as a list of all sub-pages. The pages in the menu are sorted according to the menu order specified for them. (Please see my code below).

Now, I would like to add some categories at a specific place in the menu and not sure how to better do it.

Is it possible to do it using wp_list_pages? What I like about my code is that whenever a subpage is added for a parent page that programmed to display all subpages, the newly added subpage will automatically appear in the site’s menu. As opposite to using Appearance->Menu functionality where a user will need to add a newly created subpage to the menu before it will appear on the site.

I might be easily over complicating it(!) and the only way to do it is via Appearance->Menu functionality. I would really appreciate any help and tips!

Here is the code that I use to display pages, some with all the sub-pages and some only at the top level.

//main nav: use wp_list_pages to display cirtain parent pages without any and with all child pages (a tree with parent)
$parents = array(5,7,17,61,19,25);
$children = array();
foreach($parents as $parent) {
  $child_pages = get_pages( "child_of=$parent" );
  if($child_pages){
    foreach($child_pages as $child_page){
      $children[] = $child_page->ID;
    }
  }
}
//add all the pages that need to be included without children
$parents = array_merge( (array)$parents, (array)array(31,23) );
//merge $parents and $children
$menu_pages = array_merge( (array)$parents, (array)$children );
$menu_pages_str = implode(",", $menu_pages); 
?> 

<ul class="menu wrap">
  <?php wp_list_pages( "sort_column=menu_order&title_li=&include=$menu_pages_str" ); ?>
</ul>