Adding sub menu to Management menu
Go to admin/structure/menu/manage/management; you'll see all the links currently in the menu.
Click "Add link"
Fill in the title, path and any other relevant data.
Click "Save"
Re-order the menu items if necessary
Repeat the process for as many links as you need to place under the new top-level item you just created.
If you're using the Admin Menu module, you might need to flush caches before changes take effect (but you might not).