What you need is a "wireless bridge" - where you plug a wired ethernet device into a wireless hub/switch which connects wirelessly to your main router.
I last set up something like this 2 years ago. At the time, dedicated wireless bridges were stupidly expensive, but the Linksys WRT54G could be flashed with a replacement firmward (DD-WRT) which would allow it to work as a wireless bridge. The WRT54G has since been redesigned, so I don't know if it's still compatible with DD-WRT. DD-WRT supports a lot of differnt routers, though, so there's a good chance you can get something off the shelf and modify it.
Off-the-shelf bridges may have come down in price since then, too, so that might an option as well.
Did not see your suggestion for dd-wrt, however I do know that the WRT54GL is compatible. The "L" stands for linux. That model brings more ram than the WRT54G.
In fact, there is also a version of the WRT54G running VxWorks rather than linux. That one is more complicated to flash (need to use TFTP and a "VXworks killer"). It also has a very limited amount of RAM. Because of the limited ram you can only run the scaled down version of dd-wrt. Additionally, the antennas on that one are not upgradable.
I have owned all 3. The old original one is still working well. The new WRT54GL works well as a bridge. I bought the WRT54GL to replace the VXworks version which I killed the wireless on by plugging in the wrong voltage "Wall Wart".
