You dont have to add the domains to httpd.conf file. When you set dns to point to the server, Apache will direct the page to server's default page if the domain is not added to the httpd.conf.
You can use PHP to show what domain is parked. Modify the servers default page using this little script:
<?php
if (!empty($_SERVER['HTTP_REFERER'])) {
$refe = $_SERVER['HTTP_REFERER'];
} else {
$refe = $_SERVER["HTTP_HOST"];
}
echo "Domain {$refe} is parked here.";
?>
Not the most sophisticated one but will do the job.
Oh, you dont have to use the servers default page if you dont want to. Add IP to httpd.conf that will handle all the domainpark domains and then when you add DNS to some domainpark domain, point it to that ip.