The simplest way would be to create a symlink in moo to global.php and to forums/admin.

Or:

So i put that line before requiring global.php right? ^^

I don't know if that will work, but worth a shot.
I thought changing the include path would work, (ie ini_set('include_path', '.:/path/to/script/here/'); but on *nix boxes apparently is doesn't work with files included within an include. :(

$vbpath ="/path/to/folder/with/global/in/it";
chdir($vbpath);
require_once("global.php");

or like NTLDR said