Hi,
The comment system is indeed something part of HikaShop. It is displayed by the file "form" of the view "vote" that you can edit via the menu Display->Views.
There is no common comment API on Joomla. Everyone does his own stuff.
So you can't plugin some pieces here and there.
Note however that HikaShop is already compatible with JComments. Simply turn off HikaShop comments system, and turn on the jComments radio of the 'Comments enabled on products' option of the Display tab of the configuration (JComments need to be installed to see it) and jComments form will display automatically on the product page.
You already downloaded the ReCaptcha plugin that we provide. So I don't see why you ask your last question since you already know that it is a plugin, or am I missing something ?