Yes, that's it. You don't say which Arduino. i only know the Uno (mega328). If you have a Uno, and are connected via USB then you are using pins 0 and 1 which are the hardware serial port pins. So set up a software serial port, on 2 different pins, and connect that to the other device. You can then send and receive from the other device, have the Arduino act on received data and relay to the PC.