[Lionel]

That's very easy to do actually. Create a plugin at register_addmember_complete and put this in it (replace YOURADMINID with the id you want to be a buddy of every one)

$db->query("
UPDATE " . TABLE_PREFIX . "usertextfield
SET buddylist = IF(buddylist <> '', CONCAT(buddylist, ' ', 'YOURADMINID'),
'YOURADMINID')
WHERE userid = '$userid';
");

I also made sure that no one can remove admin as a buddy