You can but it has to be set up like this.

Depending on how you have it set up, the Ethernet port can function as a LAN port (defined as either simple bridging or sharing your Internet connection via network address translation) or a WAN port (connecting to your broadband Internet service provider or upstream router). However, it never acts as both simultaneously in the way that the original AirPort Base Station (Graphite) can. When the AirPort Express is using network address translation, the Ethernet port acts as a WAN port. When it is set up as a WDS remote station, it can act as a LAN port.

Hope this helps.