Hi,
What I mean is, you can use another extension which will manage the files under modules for example.
Then in HikaShop, edit the view "product / show_default" which is the view displaying the product page.
And in the code, add the call to the file management module thanks to the Joomla tag
{loadmodule XX}
By this way your file manager will be displayed inside the HikaShop product page.
Depending on how is working the file manager, you can potentially use the custom fields to specify a file id in order to have a dynamic display of the module content. That could require some development.