Adding custom submenu to menu item

Hi everyone.
I’ve created a navmenu in wordpress navmenus area;
and it has some itmes like this :
item 1
item 2
item 3
know, i want to add a custom item as submenu to the one of menu items above like:
item 1
item 2 > sub item 1,sub item 2
item 3
I read how to adding custom item to navigation menus by adding filters in functions.php
but it adds an item end of the menu and after all items of menu and not as a sub menu item.
Please help .
Tnx.