You can do it in this way: Create a new blank action in any controller and view for it. Then render your widget into the view of this action. After that call smoothbox modal window to show needed action with your view which contains your widget. Example of how to show smoothbox by clicking a link below:
<?php echo $this->htmlLink(array(
'route'=>'default',
'module'=>'activity',
'controller'=>'index',
'action'=>'viewcomment',
'custom_param'=>$param,
'format'=>'smoothbox'
),
$this->translate('Comment'),
array('class'=>'smoothbox',))
?>