Get yourself an additional USB Wifi device, put it in Access Point mode, set up internet connection sharing and have it share your buildings internet with your Wii. Might be tricky to setup but this will work. The Nintendo USB wifi adapter does a similar thing, just at a much higher price. USB wifi devices are cheap as chips on ebay.
I think it is possible, with some routers, to have then act as a wireless relay station but not all routers support this. Your PC would need to share the incoming wireless internet connection with the LAN port.