There is an easy expensive way and a less easy, not much less expensive way.
Easy expensive way is to buy a wireless bridge and connect the output to the input of a wireless router. Linksys WET54G to a WRT54G, for example.
The slightly more difficult, slightly cheaper way is to get a router that can be upgraded with third party firmware. Perform that upgrade, make the router a bridge. Connect the output of that bridge to the input of a wireless router. Connecting a hacked WRT54G to a standard WRT54G.