In short, yes it is possible, and not very difficult.

While it is very easy to setup WDS with the Airport Express (AX) if you are using an Apple Airport Extreme base station as your primary base station, it is not as easy when using a third party base station like Linksys or Buffalo.

While Buffalo supports WDS out of the box, Linksys needs a firmware upgrade to one of the latest non-official varieties. There is a good thread in the Apple Discussion forums here. This thread applies to the Linksys WRT54G.

The basic idea from the thread above is that for the AX to act as a relay station in WDS, the settings must be exactly the same and you cannot use WPA authentication. That means that you will not be creating a new wireless network in your location, rather your existing SSID will just gain a wider coverage area.

Settings – in the WDS section of the Buffalo, I entered both the Airport and the LAN MAC addresses. If you are using the MAC filter, you should enter the MAC addresses in that section too.

Then on the AX, in the Identity section you can label the AX whatever you want, but on the ‘Airport Network’ section, you want to ‘Create a Wireless Network’ and then enter the SSID of your WBR-G54, set the same channel, and then the security settings should also be the same. You may need a ‘$’ before the password in some cases. See the Apple Discussions thread linked above for more information on this.

On the WDS tab, set the Buffalo Wireless MAC address. This is also the ‘Main Airport ID’ and the setting should be ‘relay base station’ and you should check the box to ‘Allow wireless clients on this base station.’

You will also want to change the settings on the ‘Music’ tab to suit your tastes.

That’s about it. Good luck.