by Rik Strobbe.
I just gave it a try and solved it myself:
in the file auth.php (in moodle -> ath -> db) I had to add 2 lines:
require_once($CFG->libdir.'/authlib.php');
require_once($CFG->libdir.'/adodb/adodb.inc.php');
require_once('update.php'); // added by RS: required for updating the moodle external authentication table
...
$extusername = textlib::convert($username, 'utf-8', $this->config->extencoding);
$extpassword = textlib::convert($password, 'utf-8', $this->config->extencoding);
DB_update($username); // added by RS: update the moodle external authentication table
update.php is a small script with a function DB_update that based on the username finds the required user data in the different tables and updates the external authentication table.
Rik