How to add a complete menu as an item in other menus? Let say I have 3 MENUS:

MENU 1 with these items:

item menu 1 - a
item menu 1 - b
item menu 1 - c


MENU 2 with these items:

item menu 2 - a
item menu 2 - b
item menu 2 - c


MENU 3 with these items:

item menu 3 - a
item menu 3 - b
item menu 3 - c


I need to add MENU 1 as a clild in MENU 2 and MENU 3 to get this:

MENU 2 with these items:

item menu 2 - a
item menu 2 - b
--item menu 1 - a
--item menu 1 - b
--item menu 1 - c
item menu 2 - c


MENU 3 with these items:

item menu 3 - a
item menu 3 - b
--item menu 1 - a
--item menu 1 - b
--item menu 1 - c
item menu 3 - c


Is there any way to get this?
There is a sandbox project for combining menus: https://www.drupal.org/sandbox/klickreflex/1989788

Someone with a similar issue found code that was of value.

The poster also suggests Advanced Menu which has some merging capabilities.