I do not know if there is an HID conversion available but there is an easier (and cheaper) option for better lights on X308s.
Have a look at the Phillips website.
They make ranges of uprated Halogen lamps for our cars and others.
The website has an excellent search function which will show which lamps and fitment are available for any car, they are properly tested and approved for each car, and are made to fit.
I personally do not like Xenon lights so I fitted a set of "Phillips X-treme Power" lamps to all the headlights on my car and they are superb.
Phillips say they are the highest output lamps that can be legally used on the road.......and I believe it !.
They are very powerful indeed, put up to 80% more light onto the road, and have exactly the same fitting as the standard lamp.
The only downside is a somewhat shorter life than the standard lamps because they burn hotter,while using no more current.
The best price I found was from Autolamps-online.