If you could make Drupal append a directory slash, e.g.

<http://domain/path/dirname/>

then HTTrack would produce what you want by default.

Another possible solution would be using "Options > Experts > Rewrite links" =
"Original/Original" (I think).  This would retain the links as they are on
your site, and as long as your webserver can resolve "/path/dirname" to
"/path/dirname.html" then it should work.  Apache can do that I believe.