I dont know much about online gaming, but im sure there would be some sort of audio mixer out there that you could use! It sounds like it would be possible to achieve what you are after. There are all sorts of usb audio interfaces and mixers out there these days. You essentially need to split the audio right from the mic and headphones, down to the output to the speakers, so an audio mixer would definitely be the way to go.
Sorry i cant provide a recomendation, but PC audio is not one of my strong points in the audio world..