Just chiming in off what stevendmc said. The correct way to add HID's is with an HID projector. Regular halogen headlights mixed with a HID bulb cause the light to scatter all over. The projector, does just what its name says, projectects the light in a more uniform,wider,and usually further light throw.

The aftermarket lights you see on AmericanMuscle etc, have halogen projectors. You can add an hid bulb, the results will be better then throwing them in a regular halogen headlight. But still will not out perform a HID bulb in an HID projector.

See sig for HID bulb,inside HID projector.