Im doing this.
Basically, get the following:
1x - fexible mic with a 3.5mm stereo connector from Amazon
1x - 10-15ft (or however long you want your mic cable) stereo extension (M/F or M/M)
. Im ordering a slim cable hoping to be able to tuck in down the same opening used for the headphone cables instead of drilling a new whole for that. As long as one side is Male you will be chopping of the other so dont worry about getting a Male to female. Just dont get a Female/Female extention
1x - Chassis Mount 3.5mm Mini Jack Connector
You want a panel mount so you dont have to worry about gluing it to the headphones. Just tighten the nut onto the jack.
1x - sleeving if you are wanting to hide the 2 cords to make it a cleaner look. Otherwise just use zip ties.
You also need soldering iron, solder (and basic soldering skills) and drill. I havent see anything around online yet so I may make a start to finish tutorial about this while im doing it.