ISPConfig does not create the folders within Maildir. It just tells Postfix/Sendmail to deliver emails to Maildir, and when the first email arrives, then the folders are created.
But you can use the command
maildirmake (see
man maildirmake or
http://www.die.net/doc/linux/man/man...dirmake.1.html ) to create custom folders. To do this automatically, you can change the function user_insert() in
/root/ispconfig/scripts/lib/config.lib.php.