The ErgoDox uses the Teensy 2.0 and MCP23018, so every firmware that supports the ErgoDox should contain code for them in some form or other.
QMK does not have a code module with a generic API for I²C to the MCP23018. You will find it in the source code that is specific for the ErgoDox EZ, and you would probably have to copy and paste code from that and rewrite a bit for your keyboard.