Drop Down Sub-Menu For The Menu?

Hi all, i would like to add sub-menu as dropdownlist in the right menu of colum2 layout but don't know how to do it.

$this->menu=array(
	array('label'=>'List User', 'url'=>array('index')),
	array('label'=>'Add Skill', 'url'=>array('user/skill'),
              'item'=>array(....)// would like to become dropdownlist or sublist
              ),
);

Please give me any links or solutions for this question. Thank in advance