NB most I/O expanders only provide digital output, IE on or off, for a fan you really want PWM so you can control the speed.
First you have to identify the chip used on your PWM capable IO expander, then find an arduino example of how to use that chip, finally you integrate that into the marlin firmware.
If no one has made it work on arduino, then you have to find the datasheet for your chip, read, it, understand it, let it take over you life, and write code to use it.
"16 Channel 12-bit PWM/Servo Driver-I2C interface PCA9685 " should work... and your still going to have to add mosfets
Adafruit has libraries [learn.adafruit.com]
you just need to modify the M106/M107 commands as needed
First you have to identify the chip used on your PWM capable IO expander, then find an arduino example of how to use that chip, finally you integrate that into the marlin firmware.
If no one has made it work on arduino, then you have to find the datasheet for your chip, read, it, understand it, let it take over you life, and write code to use it.
"16 Channel 12-bit PWM/Servo Driver-I2C interface PCA9685 " should work... and your still going to have to add mosfets
Adafruit has libraries [learn.adafruit.com]
you just need to modify the M106/M107 commands as needed