Actually – you can extend a non-Apple wifi network if you use 2 airport units, or an airport and a Mac.
– Using 2 Airports – 1. Configure Airport 1 to join the wifi network 2. Configure Airport 2 to Create a Wireless Network 3. Connect Airport 1 client port to Airport 2 WAN port with ethernet cable
Caveat: I think Airport 1 has to be an Airport Express. Configurations for AIrport 2 Base Station: Any Base Station name and password Internet: DHCP Wireless->Network Mode: Create a wireless network -> Wireless Network Name: your SSID -> Wireless Security: WPA/WPA2 Personal -> Wireless Password: your password Wireless Options -> 5ghz network name: Another Network Name Country: Your Country -> 2.4ghz Channel: Automatic -> 5ghz Channel: Automatic Network: DHCP and NAT
When both airports are working correctly, their LEDs should be green.
– Using a Mac and an Airport – 1. Connect your Mac to the Wifi network. 2. Configure your airport to Create a wireless network, as above. 3. Use an ethernet cable to plug the airport into the Mac. You may need a USB->ethernet adapter. 4. Configure the Mac to use “Internet Sharing” FROM: Wi-Fi TO: your ethernet connection to the airport.
That’s it, your airport should be broadcasting a WiFi network that other devices can join. The airport will route to the Mac to use it’s wifi. The Mac is still completely usable and can use it’s own WiFi connection to the original network.