or actually you could change the source code and submit your change to the plugin developers source code within github (most of the e-commerce plugins have a github open project)
so how to find where in the plugin you should change the supports line...
on woocommerce plugin i did that, that way:
while using sublime text editor I've searched all the files for the occurrence of the phrase:
and that found me a couple of results but on the second result (the first find was a comment) I`ve got the code that declares new custom post type called "product"
so the string is in woocommerce/woocommerce.php file (in your plugins directory)
on line 982
so just after
add the following in the same line:
and save the file and upload to the server.
And that is it.
you've added to the 'product' custom post type declaration the option of comments in this custom post type.
<em>a couple of foot notes:</em>
your are changing the source code of the plugin.
the next time the plugin goes for an upgrade all your changes will be deleted, and you will have to do this process again from the start.
I think there is a way to re-declare the product custom post type with the new added support option, with calling this function with its attributes:
but i suggest that everybody should check that option only if they already know how to write php arrays and call functions...