I2S will probably be a soldering hack. Do a google for I2S Gert soldering and you'll probably find it at some point.
KNX I know nothing about, so I hope someone else can answer that.
Modbus: Depends. I know there's a serial and a ethernet spec, so it kind of depends on which you mean. Modbus over ethernet will work, and there's plenty of Python code available on the internet to talk with the PLC's. It works, but timing can be fiddly if you use to serial port module on the modbus to talk with something time-sensitive. As for serial, you've got UART, which can be converted to RS232 with the appropriate level converters. However, you haven't got hardware handshaking, there's no RTS or CTS brought out onto the GPIO. You can get a normal USB to RS232 dongle, most work with Linux.