Thank you, I will study this. But it seems quite complicated. Still it would be the best solution.
My goal is to replace a daughter board on a existing device (with a raspberry Pi as the main MCU) with an Arduino that acts simply as a USB/serial converter. For mechanical compatibility of the daughter board I dont want any FTDI USB/serial converters. In fact, the daughter board with the Arduino is only a USB/serial converter that interacts with another remote Arduino that controls electro-mechanical relays.