@ seyedmiri,
As you might know, JA Smashboard is a special template that our devs override the layout of Joomla menu module and used the javascripts to display the current scrolling navigation, instead of using the Mega menu like other T3 templates.
So, it will need really heavy customization to modify the menu layout again to get your requirement with sub-menu items. I would suggest you hire or corporate with an experienced developers to help you handle this task.
For your reference, you will need modify the overridden menu module layout in templates/ja_smashboard/html/mod_menu/default_component.php file. In addition, the script.js file in templates/ja_smasboard/js/ also contains the majority of defined functions on scrolling effect of navigation.
