If you've got anything other than sealed lamps your lighting already isn't stock.
If you upgrade to HIDs make sure the enclosures you get have a decent cut off (Like Cibie or even Hella's E-Code H4 housings)... otherwise you're probably more likely to get hit from just blinding fellow motorists or get ticketed.
Also check the laws in your area, aftermarket HIDs are not legal in many places.
EDIT: Oh, and no, you can not use the stock wiring to power the ballast. You may use it to power a relay though.