Thought about adding rotary encoders to this unit too? From what I can see it’s the same unit as mine, and if you are only using 16 buttons you could add up to another 8 encoders. It’s relatively easy and if you’re looking for some help, let me know.
Yes, you’re correct. You take two inputs from the Leo Bodnar controller and one of the grounds. Remember to download the encoder drivers from the Leo Bodnar site, I’m working away from home for the next few days, but I will try and find them and post the link if you do decide to put them on your unit. Look on Ebay for rotary encoders and you might find a batch of 10 relatively cheap (£10-£20). *edit - actually the encoder configuration is on the front page of the unit website about half way down - BU0836X joystick interface : Leo Bodnar Electronics
Remember, if you want to add push button encoders you will have to allow an input for them too.