This board has a TTL UART input in the corner just aside the USB connector. Why would you use USB serial emulation if there's simpler solution available. You have a rs232- and a TTL-level interface available, I'd use that.
To answer your question: You could use your UNO to do that but you have to compile a new firmware for the Atmega16U2. To my knowledge no software exists to terminate a USB serial connection as a host on that chip (the other side is standard in the UNO), so you have to write your own and optimize it to fit into the 16kB flash of the 16U2, not a trivial task.