Hi All,
In the end, to fix the issue with the Fuel Additive Tank I had to buy a new Fuel Additive Tank. I did not find a way to cheat the system with resistors or things like that.
Unfortunately, I did not find a garage to replace the faulty additive tank as the job has been described as massive. Anyway, I find a workaround which I hope could help someone else: I connected the new Fuel Additive Tank in parallel to the Fuel Additive Module. I cut the sensor wire (yellow one) from the faulty FAT and the motor wire from the new one (green/black one) and put the new FAT in the boot.