Hi, Welcome to the forum!
Most plotters are too whimpy to run a dremel. To convert it is more work then building a whole machine. The machine pretty much needs to run on Gcodes and not HPGL. HPGL has no provision for a z axis. It has pen up/down but your plotter will be moving before your z will be down.
For plans for inexpensive machines go to
www.crankorgan.com
Good luck, you will love this hobby!
Eric