There are universal wireless repeaters that you can purchase. That's what I have done at my brother's house several years ago. But now not I do not like having a specific device for one function only
Last year I experimented in replacing existing router firmware with the open source dd-wrt firmware (dd-wrt.com). It has specific versions for different routers. Basically it replaces the router's firmware and gives it advanced features such as universal repeater and repeater bridge functionality; and increasing transmission power, and encryption which is what you are looking to do.
I've used this on several Linksys WRT54GL routers and even on a Belkin Wireless repeater device. The MAC address filtering does not seem to work in repeater or repeater bridge mode but the WEP/WPA encryption schemes will work.
If you go the dd-wrt method please see if your router model and version number is supported, then read faq, forums carefully since you are essentially replacing the manufacturer firmware (invalidating the warranty) and have the potential to "brick" the router.
You can also ask me some questions before you proceed, since I'll also be updating the dd-wrt version at my parent's house when I go on holiday next week.