as far as I'm aware you don't need to do anything on XenForo's side. If it's in /library/Zend - simply include it in your script.. before you extend it.. like so
PHP:
require_once('/library/Zend/Service/Twitter.php');
function MyTwitterLoginScript extends Zend_Service_Twitter {
// stuff
}
As far as I know, the entire ZF is packaged with XF anyway. Looks like a pretty complete set that I'm looking at.