Hi,
HikaShop already has add to cart buttons, a mini cart, etc.
What the templates for HikaShop do is that they customize these elements.
So if your template doesn't have these customized elements, then these elements will just be displayed with the default display of HikaShop.
So for example, if you want to use a template made for VirtueMart with HikaShop, you can, and all the functions of HikaShop will work. However, the add to cart buttons, the mini cart, etc will display not like what you see on the demo website of that template. Instead, they will display like they would display with the default template of Joomla.