A headphone that has an USB connection means it has a built-in DAC (a.k.a. miniature sound card). that will take the PC's digital data and convert it to analog sound. A 3.5mm connection is only good for good old analog sound. So if I am guess right, you want an 'adapter' that will make your headphone USB connection takes an analog sound via a 3.5mm connection. The short answer is, such magical adapter doesn't exist, because it is impossible to ask the DAC inside your headphone to recognize the analog signal that sent to it, even if someone can hack an adapter together for you. It will be like trying to stuff a floppy disk into the CD-rom of your PC and ask it to play it out.
Cutting off the USB connector might not work (that easily) as well. I reckon the DAC is inside the earcup and there might be some amp section too. If that is so, the speaker driver inside the headphone might not have enough sensitivity to sound loud enough without some kind of external amplification. That's just a lot of work for very little gain, unless you are the DIY type and like to take on a project.