It's easy to do this for question pages. Modify the function qa_q_request() in qa-base.php to change the last line from:
return (int)$questionid.'/'.implode('-', $words);
... to ...
return (int)$questionid.'/'.implode('-', $words).'.html';
Doing it for other pages is a lot harder but probably less important if you're concerned about this for SEO reasons (not that I think you should be!)