SebastianMWS,
I bought a Noctua 4-pin PWM fan, filed down the two plastic sliders on the 4-pin contact so it fits nice on the PCM board in the power supply. That´s all i had to do to make it work. It seems that neither the power supply nor the motherboard monitor the fans in the power supply. The fans rotated and the air moved from the front through the power supply and out from the back, just as the original fans moved the air. No warnings from the BIOS.
The original fans have two plastic sliders as well but are wider apart from each other and that´s why i needed to file them down on the Noctua fans. Place the filed end the same way as the original fan contact are.
If i do the same and attach the fan to the motherboard the BIOS warned me at every boot. Pressing F1 to continue was required. That´s why i opted for HP fans for the chassi fans.