You can absolutely use external pedals with the Jamstack 2 with some limitations in placement. You can either have the pedal(s) at the start of all of the internal pedals inside the Jamstack 2, or after. You can't place them in the middle of the chain.
Here we show a Wah pedal at the begging and the end of the chain and which cables you would need to make it happen.
Ideally you would put it at the beginning of the chain to keep stereo output (which you would do with a Wah pedal most often anyways). You can put pedals at the end of the chain, but most pedals only output a mono signal, which is only really an issue with stereo delay.
