Show subpages of a category in a custom menu

If you created a menu, you would have to drag in those pages, as the basic Walker_Nav_Menu class does not pull subchildren into it. If this is your desired effect, you could created a custom Walker, essentially duplicate the start_el() function of Walker_Nav_Menu, and perform a check on $item->type to see if you are dealing with a Category – then create your submenu manually.