Hi @HammyHavoc, welcome to the forum.
There are several ways to add I/Os to the Zynthian. One, using the GPIO pins is discussed here: Extra switches now customizable from webconf It allows you to add things like switches and pedals. The other option is very new and allows you to add CV/Gate I/Os. It is being discussed here: CV/Gate input/output
Both these topics are works in progress, but my experience is that @jofemodo and the Zynthian team are pretty amazing at developing and incorporating new ideas as they come up.
Hope this helps.