If the old htm files have a commonality in the links that you need to convert (as in www.oldsite.com/page100.htm needs to convert to www.newsite.com/page100.htm ), then you could load the site into an editor and do a mass search/replace of ‘oldsite.com’ to ‘newsite.com’. (I would use my copy of Dreamweaver, which as a search/replace across files in a folder.)
But that would require putting the page100.htm in a non-WP location.
You could create a template that did a ‘list files’ of a folder and convert those to HREFs. But that would only get those HTMLs on the site.
If you need to get the old HTML content into WP, you could write a manual conversion program that reads each HTML file and creates a post with that content. That doesn’t take care of links in the content though. For that, you’d need to write a 404-not-found process that would help the user find the content via a search function (“We didn’t find that page, but here are some pages with similar content”).
I suspect a lot of custom programming and labor to do this. Although more details of the problem (with a few examples) might be helpful. Interesting question, though, since I face a possible similar problem on a site that I manage, so look forward to other’s thoughts.