Basically you will have to replace the template "welcometext" with PHP code within index.php.
Your code will then have to check if they are a valid user by checking the $bbuserid variable against the database. If they are a valid user then use the default welcometext string. If not use the string you wish for the quests.
I.E.
Code:
if ($bbuserid) {
...*Check Database Here *...
$welcometext = "Welcome Back " . $username;
}
else {
$welcometext = "Your a guest go register please!.!.!"
}