I finally found a way.
Adding these codes to the file inc/preload.php
where ABCD is the location base of the wiki site you want to inherit from
$config_cascade['plainauth.users'] = array(
'default' => '/ABCD/conf/users.auth.php',
But, my blogging plugin now show errors because "SQLite & PDO SQLite support" is missing. Not sure this is the consequence.