The only way I know is direct creating of Users on the DB.
The way I use is:
--
-- Daten für Tabelle `users`
--
INSERT INTO `users` (`user_id`, `username`, `mail_host`, `alias`, `created`, `last_login`, `language`, `preferences`) VALUES
(USERNR (1.2.ETC), 'MAILUSER', 'MAILSERVER', 'LOGINALIAS', '0000-00-00 00:00:00', '0000-00-00 00:00:00', 'de_DE', 'a:10:{s:16:"message_sort_col";s:4:"date";s:18:"message_sort_order";s:4:"DESC";s:8:"timezone";d:1;s:10:"dst_active";b:0;s:8:"pagesize";i:36;s:10:"prettydate";b:1;s:11:"prefer_html";b:0;s:10:"htmleditor";b:0;s:12:"preview_pane";b:0;s:14:"draft_autosave";i:300;}');