I have Corsair 1500 headphones that I use with my PC through USB. Is there a way to use them through 3.5mm for use with my phone/Xbox 360?
doesnt seem like you will be able to.
What you're asking for is an ADC (Analog to Digital Converter). However, it probably won't work because the headset will still need the power that it gets through USB, unless your headset doesn't have an amp or add-on effects, which I doubt.
As for your phone, you might be able to take a USB to micro-USB adapter and use that - I don't know if it works with phones, though, and that's still not going to be adapted to the Xbox.