Fan etc on external i2c-controller

Hi there,

my Board (GT2560) does not support controle of more then one fan.
So I would like to add I/Os by using an i2c-expander.

I know, that I have to modify the GT2560-Board to reach the i2c-Bus - but how can I teach Marlin to use the new I/Os for the fans?

Thank you in advance for any help!

Best regards
Maeckes