Is it possible to use another menu as a submenu of a menu? Is there a setting or a module, this would facilitate re-use of menus...

For example:

Menu 1
  option A
  option B
  option C
  option D

Menu 2
  option 1
  option 2
  option 3 
  option 4

Full Main Menu
  option A (use menu 2 for sub menu options)
    option 1
    option 2
    option 3 
    option 4
  option B
  option C
  option D
There are not settings I am aware. I have not done an exahaustive search for a module. However, you could write your own implementation with a custom module implementing hook_menu(). Simply put, you would create a new menu. Then, you would iterate through the other menu's in your implementation adding each element to the new menu.