The only reason I didn’t buy that HP pen plotter is because I’d want the freedom to give it any kind of pen or marker, not just ones made specifically for it. I imagine it’s not possible?
You can make your own adapters depending on the pen you want to put in it. I've made some out of dried out HP pens that I cut the ends off and jammed a Sharpie into. I've seen nicer 3d printed adapters on eBay and Thingiverse as well. When using a heavier pen like a Sharpie, you might want to set the plotter's speed a little lower to avoid knocking the pen loose when it accelerates.
