Replace solid axle with quick release on Fixed Gear
This is easily accomplished, simply replace the axle from your wheel with a QR axle, at most you may have to cut an axle/skewer to length and maybe cut more threads if you can't find an axle and skewer with enough thread on it. But you'll need to be comfortable with cutting metal and have the right die set. 

 

If you try shopping around for a 120mm rear QR axle, you'll have better luck looking for a complete hub and just parts it. They're not super common though, and not cheap...