People often use Arduinos to send data over serial to other peripherals (flashing firmware onto modules, etc) by pulling RES to ground, connecting the TX and RX pins to the device being programmed, and then sending data to the Arduino over serial. As an alternative, one could also remove the ATmega instead of grounding RES.
That being said, there should be no reason why one could not receive data from a connected device in the same way. However, no matter what I try, I never succeed in using an Arduino to relay serial input from another device over USB.
Is it even possible?