There may be a better way.

Export site
"index" will be there. manually type ".html" for "index.html" and Save.
that will export the entire, but the default is still .htm when I just tested it.
That means you'd have to manually add the ".html" to "index" every time you export the site.
Not THAT bad, but I couldn't find a way to change the default extension. Someone else may know how.