Firstly meta keywords have absolutely no effect on SEO whatsoever.
However if you definitely want to do this you will need to do a small "core edit". In qa-page.php, look for this code around line 640:
if (!empty($qa_content['keywords']))
$themeclass->output('<META NAME="keywords" CONTENT="'.$qa_content['keywords'].'"/>');
// as far as I know, META keywords have zero effect on search rankings or listings
Change it to this:
if (!empty($qa_content['keywords']))
$themeclass->output('<META NAME="keywords" CONTENT="'.$qa_content['keywords'].'"/>');
else
$themeclass->output('<META NAME="keywords" CONTENT="your keywords here"/>');
Hope that helps!