How to adapt the extended IO?

What sort of expansion module? I2C? SPI? something else?

You might need to write a HAL driver.

Do you need this IO to be realtime?

There are tutorials on the web about enabling I2C on the Pi (for example learn.adafruit.com/adafruits-raspberry-p...etup/configuring-i2c )

For non-realtime access you can use the built-in drivers. I think that for realtime use you would need to mmap the device. I suggest looking at the i2C kernel driver tutorials out there (but don't follow them) and look at the hal_pi_gpio LinuxCNC driver and try to combine the two.

You can probably write this driver using halcompile.

If you can find a way to read / write to the board using inb() and outb() then it is easy to write the HAL driver. (That is how the paralell port driver on x86 works)

hal_pi_gpio wrks somewhat similarly, except that it uses mmap to allow it to read the port data from (virtual?) memory locations:
github.com/LinuxCNC/linuxcnc/blob/master...rivers/hal_pi_gpio.c

I think that googling for Pi I2C and mmap might get you the required info.

It might well be as easy as changing the /dev/gpiomem here:
github.com/LinuxCNC/linuxcnc/blob/master...s/hal_pi_gpio.c#L164

to /dev/i2c-1 (or i2c-0)
raspberry-projects.com/pi/programming-in...ng-the-i2c-interface

I think that I would suggest starting with a _very_ simple HAL component written with halcompile that first of all reads the data into a single u32 HAL pin using the sample code above, then look at memory-mapping that port, and only then worry about setting individual bit-type HAL pins and configuring port directions.