If your new fans can do 5k RPMs then just plug them in and the miner will be happy, not sure what do you want to "disable" the fans all together unless you were trying to liquid cool them, in that case you will need fan simulator found cheap on Amazon.
If you just want to lower the fan speed and put a static/fixed RPMs, I wrote a
topic on how to do it, it's easy and straight forward.
There is no known and trusted custom firmware for the S9k, but thierry4wd unlocked the stock firmware, if you have the programming skills needed, you could probably disable the fan-check function.