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
) to create custom folders. To do this automatically, you can change the function user_insert() in /root/ispconfig/scripts/lib/config.lib.php