I made my own roof rack, and I got the Hella's through ebay from a Hella dealer here in Georgia. There is a Hella factory in peachtree city, GA, and they sometimes have public sales.
The housing conversion is simply drilling a 1 inch hole in the backside to allow the fitment of the rubber grommet from the HID kit. Other than that, its nothing to it. There is no other 'conversion' process.
Here are a few shots from when the same lights were mounted on my old yota to give you an idea of how they work.
From inside the truck, just standard low beam headlights.
Outside the truck, same spot, same low beams.
Inside truck, same spot, with the (4) 4000 HID's on. They hadnt been aimed yet. The treeline in the back ground was 1/3 of a mile away.
From outside the truck.
This is broad daylight, about 1pm.
I went with the 5K bulbs for the whitest/brightest light.
Let me know if you have any specific questions.
~James