How much are you willing to hack things?
Find a retractable ball-point pen made from hard plastic. Take out the ratcheting mechanism from the rear so that the clicker is always pushed out. Ideally, add a spring from another pen to increase the force.
Find a scrap tool of some kind–HSS or carbide. Either something really small, or with a sharp point, or even something that broke in a way to leave a point.
Attach it to the pen clicker in some way. Heat it up and melt it in, superglue, whatever. It’s an engraver and doesn’t need too much force. Try to center it, but don’t worry too much if it’s off; it’ll just have a minor XY offset as long as it doesn’t rotate.
Mount the “tool” in the Shopbot collet (1/4", something like that). The pen is plastic so it there should be plenty of tolerance for just forcing it in, but if not you can find the next size up and wrap tape around the pen until the diameter is close enough.
The pen is long and wobbly, but if you move slowly and cut only soft things (plexiglas, etc.) it should be ok. You’ll have around a centimeter of travel; calibrate the Shopbot to use around half of that. Make sure the Shopbot doesn’t enable the motor. Wear eye protection in case it does and your hackery centrifuges apart.