PCF8575 and Keypad

Hello! I am hoping to use the PCF8575 and your library on a musical keyboard project. In my Arduino sketch-in-progress, I am using this Keypad library to detect piano key presses - and when I use the built-in pins of an Arduino Uno, I can get it to work. Now, I would like to free up my Arduino pins with a PCF8575 module, but I cannot seem to understand how to tell "Keypad" to use the PCF8575 pins instead. Does anyone have any insight or suggestions? Thank you.