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extending wirelessly, probably not

wired, fairly easily
if you can get an ethernet cable to the location you want to put the express, just give the express the same SSID (name) and security settings, but different channels, your devices will then automagically switch and pick the strongest signal, you won't notice much when it does that.
 
AirPort Express can extend the range only of an AirPort Extreme or AirPort Express wireless network.


it can work, just not supported by apple, and requires a bit of tinkering and the correct hardware/software combination to get it to work (if at all), hence the "probably"

http://www.dd-wrt.com/wiki/index.php/WDS#Apple_Airport_Express

also to keep in mind for using wireless. (from the link i posted)
Also take note of the fact that all repeaters, including this WDS Repeater mode, will sacrifice half of the bandwidth available from the primary router for clients wirelessly connected to the repeater. This is a result of the repeater taking turns talking to not just one partner, but to two, and having to relay the traffic between them.
 
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