Well, out of my head, I'd say no, you can't do it from httpd.conf. However, I'm sure you can - I just don't know how! 😉 You could probably find documentation on this on mod ssl website. However, you can use a simple php function to check if you're in SSL mode or not, and/or redirect to ssl mode. Here is a function stolen from the SourceForge code:
function session_issecure() {
return (getenv('SERVER_PORT') == 443);
}
then, you could do something like;
if (!session_issecure()) {
print 'This page requires SSL mode<br>';
exit;
}