OK, let's presume that speed is not a problem. That requires a NAT table and SRAM to hold the packets that are enroute from one network to the other. How do you plan on holding the packets during the transfer? A packet could be 1500 bytes. The Uno-type processors don't have that much SRAM available after a normal startup.
And you must hack the w5100 library to get access to the raw packets on each socket.
You can always try it, but it would not be anywhere near a network router.