You have a few options for this sort of situation:
1. You can increase the strength of the signal – routers typically use wireless microprocessors (Broadcom, TI, Intel, etc). These chips are made for world markets and then they are packaged and distributed by companies like Netgear and Linksys. Laws vary country by country on the transmission strength on wireless frequencies (in this case 2.4ghz). So the chip itself is typically capable of being overdriven. My Linksys wireless router is defaulted at a transmission strength of 25mw but the broadcom chip it uses has the capability of transmitting at over 250mw! Companies like Linksys and Netgear are required by law to prevent this, but it is typically easily overcome with a little lower-level hacking. Check out http://dd-wrt.com for more info on this
2. You can get range extenders and amplified antennas for routers that will increase the transmit power. But you are still limited by FCC guidelines like mentioned above.
3. You can setup multiple wireless router in an extended range network using a protocol like WDS. This will cause multiple wireless routers to act as one.
4. You can bet a “wireless bridge” which has similar end-user functionality as WDS but the backend works much differently. The wireless bridge simply relays signals in a “dumb” fashion to effectively extend the range of a given wireless device.
It sounds like option #4 is what you are looking for, it is the simplest and easiest to setup.
Good luck!