No. It's a regulated light - a pot on the supply wires would just lower apparent battery voltage (low enough it cuts off) and a pot on the bulb wires is a bad, bad, bad idea (If you don't know the voltage problems you shouldn't be soldering on a HID!). Also, arc lights act funny on too-low current.
It has been done on a OEM bosch ballast over at hidplanet.com by modifying the actual circuit board itself, course this only turns "up" the wattage not down. This is also really dangerous if your don't know what you are doing as you are dealing with really high voltage that can kill you! Also probably shortens the life of the ballast/bulb.... would not try it with my headlights thats for sure..