The v5 brain doesn’t have a USB host port, so it wouldn’t be possible to connect a keyboard/mouse directly to a v5 robot in a useful way. There will need to be some sort of intermediary device which has a USB host port, can read input from the keyboard, and then send data to the v5 brain (via RS485, info about doing that in PROS here). As @Barin mentioned, an obvious candidate for this intermediary device would be a Raspberry Pi, since it has USB host ports and can communicate with generic serial devices.
To answer your question, yes, you would need some code to do this. The intermediary device (RPi etc.) would need to have a program running on it to read input from the keyboard/mouse, possibly process/interpret it, and then send data/commands to the v5 brain over serial. The v5 brain, in turn, would need to be programmed to receive that data and take appropriate action (i.e., turning a motor on when it hears that a particular key has been depressed, or whatever other controls you want to implement).
Again, this would not be legal for use in VRC, but definitely seems feasible.