Because I didn't need to have the user's email-adresses I just commented out the whole part of the verification of valid email adress in wb/admin/users/add.php, line 81, all the lines which you can see here:
// Check if the email already exists
$results = $database->query("SELECT user_id FROM ".TABLE_PREFIX."users WHERE email = '".$admin->add_slashes($_POST['email'])."'");
if($results->numRows() > 0) {
if(isset($MESSAGE['USERS']['EMAIL_TAKEN'])) {
$admin->print_error($MESSAGE['USERS']['EMAIL_TAKEN'], $js_back);
} else {
$admin->print_error($MESSAGE['USERS']['INVALID_EMAIL'], $js_back);
}
}
cheers,
Mux