You should be able to plug an ethernet cable into the Mac and to the router. The only way to get the signal into the router is by the Wlan port. Since the signal comes into the Mac you can share that signal with your network.
I had a similar type of setup a few years back when all I had was dial up and I was able to get Direct Tv's Direct Way Internet connection but the problem was that thier modem setup was two seperate modems , one for up load and one for down load and the connection cable frrom these modems was a single usb cable and not an ethernet cable. Also at that time there was no usb to ethernet adapter to be found. So I connected the usb cable to a laptop and the an ethernet cable from that laptop to a router and set up a network. The only thing with that setup was that the laptop had to stay on all the time for the network to function.
So far the Mac is broadcasting the signal just fine thru its Internet sharing. Now! The newest problem is getting the signal to repeat thru my Amped SR10000... Every time I try to connect it thru that, the iMac shuts off it's Internet sharing and my repeater needs resetting... Any ideas?