can I use an extra router to extend a wireless network?

This depends on what type of router you have. But ultimately you need to have the option for WDS (wireless distribution system). You need to setup your second router to not be a DHCP router and only a bridging router. At that point you will be able to extend your network. Its not too tough to do, definitely worth learning about if you have two routers.

With Linksys routers you can upgrade the firmware to take advantage of this, Belkin's have it built-in I think. D-Link has it built-in too I think, but Google can help you figure that out. Let me know if any more Q's.