Categories Dropdown Menu

Hi,

You can't put a module inside a joomla menu module.
You can have an accordion display for your HikaShop module like we did on our demo website. You can look at the module's hikashop options in the back end of the demo website and set the options of your module in the same way to activate it.
However, you can't have the products listing displayed as a dropdown with the options available. You would have to edit the file "listing_list" of the view "category" via the menu Display->Views and modify the code there to display a dropdown instead of a UL list.