Product permerlink. How do you add .html at the end of custom base?

In products permalink (i think this is woocommerce), how do you add .html to the end of every URL? When I try to do it, it keeps adding a foward slash at the end. So it would be mysite.com/product/title.html/ instead of mysite.com/product/title.html

the reason I am doing this is, because I got a CMS, that has URL structures that end in .html. And there is 15000 urls. Coverting to WP, I do not want to redirect all those pages.