Building large menu that combines many other menus

Can I use following script to build my primary menu which includes items of many other menus?

Example: my main menu looks like this -

-1
--1.1
--1.2
-2

and I have many other menus with ID = (1.1, 1.2, ......), so while building menu on a webpage it should combine all matching menus. Result should be like this:

-1
--1.1
---items from menu id 1.1
--1.2
---items from menu id 1.2
-2
and so on.

Here is a code that I have -

// Get the nav menu based on $menu_name (same as 'theme_location' or 'menu' arg to wp_nav_menu)
// This code based on wp_nav_menu's code to get Menu ID from menu slug

$menu_name = 'custom_menu_slug';

if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $menu_name ] ) ) {
$menu = wp_get_nav_menu_object( $locations[ $menu_name ] );

$menu_items = wp_get_nav_menu_items($menu->term_id);

$menu_list = '<ul id="menu-' . $menu_name . '">';

foreach ( (array) $menu_items as $key => $menu_item ) {
    $title = $menu_item->title;
    $url = $menu_item->url;
    $menu_list .= '<li><a href="' . $url . '">' . $title . '</a></li>';
}
$menu_list .= '</ul>';
} else {
$menu_list = '<ul><li>Menu "' . $menu_name . '" not defined.</li></ul>';
}
// $menu_list now ready to output
source - http://ift.tt/1xMgu0C

My aim is to break down large menu into smaller ones and then build them up on my pages.