Your models don't necessarily need to extend the Zend_Db_Table_Abstract object at all. Conversely if the class provided by Zend does everything you want except for that minor inconvenience then simply fetch the DB object in what ever method it is that you need to join on.

Also, we'd need more information on your database schema and the like in order to make a better recommendation on how to plan your models.

On a personal note, I think Zend is vastly superior to CI.