When using crossover cable to network two computers how do I connect modem Ethernet cable to original unit?

You would need another ethernet port then make that computer share the internet with the other computer. The best thing to do would be getting a switch or a router that way both computers would have internet access and the other computer would not be dependent on the other computer. It costs more but it is more efficient.

Crossover is for direct computer to computer connection.

If there is a modem involved you need a patch cable.