I have seen on the Apple discussion forum that there is no solution for the mentioned redirection. Today I have found a solution which was plain simple as all things Apple.
Every time you click on the mail link in Wiki Server it will take you to SquirrelMail login screen. This screen is the login.php file.
So in order to redirect Wiki Mail to any URL of your choice you just need to edit or substitute the login.php file which is located in /usr/share/squirrelmail/ like this:
<?php
/* Redirect browser */
header("Location: http://your_preffered_url/");
exit;
?>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120216094255785