Well, in order for you to do that, the headphones should have their own sound driver. Any conversion unit will also need that and I don't think it will be cheap.
Edit. Nevermind
http://www.amazon.com/StarTech-com-Stereo-Adapter-Converter-ICUSBAUDIO/dp/B000NPKGGK
http://www.amazon.com/Sabrent-External-Adapter-Windows-AU-MMSA/dp/B00IRVQ0F8/ref=pd_sim_147_1?ie=UTF8&dpID=51FLiQuh7EL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=0AEZZSH11B8T5JDWFF8V