What you need is a router that can be turned into "repeater" mode instead of "Access Point" mode. Without the exact model number of which router you have, it is hard to say if you can.

Even if it not a feature, many routers can use custom firmware, which can make this feature available.

Helpful questions:

- http://superuser.com/questions/19184/can-i-make-two-wireless-routers-communicate-using-the-wireless  
- http://superuser.com/questions/27720/recommended-gigabit-802-11n-routers-that-work-with-dd-wrt
- http://superuser.com/questions/672/which-routers-do-you-prefer-for-dd-wrt-or-openwrt