more GPIOs with extension boards

[kwetter]

Hi, im curious if it would be possible to get more GPIOs (to e.g. use more buttons) with extension boards. I currently have some MCP23017 and PCF8575 boards lying around, also maybe this is possible with an additional arduino? How would I go about to set up such an extension board or an Arduino and use those GPIOs with phoniebox, Is it even possible? Any tips and hints in the right directions are helpful. Thanks in advance!
I got one prototype box working with the help of @splitti 's tutorials. My goal is to have some kind of hotkey buttons (e.g. 3x3 buttons) for certain songs or podcast (similar to Hoerbert) along with the RFID keys and the next/Prev-buttons, and VolUP/Down Slider.