A rotary know will need at least two inputs and the order of signals tell you which direction the knob is turned (if you're talking about a digital one). So you could replace one 3-way-switch or two 2-way-switches by such an encoder.
If you want an analog knob (like the knobs and sliders on Devo10 and 12) you need an analog input to connect it. There are some, but mostly used or you would have to solder a wire at a very small pin directly at the processor (and care for a clean signal) (GPIO A0, C5).
