ethernet hubs don't exist so I'll clear things up a bit. When the shub is in modem mode it is just that so none of the router/switch bits work including wireless and the "lan" ports on the back of the shub. In order to extend your wired network you need a "switch" which you can connect multiple devices to. Seeming as though the airport express only has two ports on the back one will need to go into the shub and the other one to a switch such as this
http://www.amazon.co.uk/Linksys-8-Po...gigabit+switch You can then connect any wired device you want to the switch and it will handle the traffic accordingly. (if you dont need 8 ports you can get a 4 or 5 port switch)