As said by Villian, light mods can affect the actions of other drivers because if not installed correctly the person won't be able to see and could possibly crash.

Something like adding a noisy exhaust, sure it's noisy but it's not affecting other drivers ability to drive properly.

Anyways, this wasn't a legality question so..
Yes it's possible to install HID's it's simple all you have to do is buy the parts and it's pretty much plug and play.
You also have to make sure you get the cut-off right.

It has been done by a couple of people.
Out of those people I think only one has installed projectors?

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what do you mean about "getting the cut-off right"?

with projector headlights there is a cutoff. with the cutoff, there is no light above the cutoff. letting more light shine onto the ground. That is the design of a projector as apposed to a halogen