http://www.usbwifi.orcon.net.nz/ Do-It-Yourself Wireless Antennas Update and Wi-Fi Resource Center | WiFi Wireless Frequently Asked Questions
here's a few interesting pages for you which give different routes you can go down. as long as your wireless router has a repeater function (to enable it to act as a wireless range extender so to speak) and an external antenna connection it should be possible to attach any type of antenna, but the usb options are the cheapest.