I missunderstood your initial post. I though you were asking, and not giving info. My appologies.
But i still do not know how that would work, because you would be using the 360's analog left and right rca cables that would hook up into that box converter, of which the 5.1 PC headphones would hook into. Can you even enable 5.1 digital in the 360's setting when you don't have a TOSlink cable hooked up to it?
With a docoder, the best i believe you could get is Dolby Pro logic II (fake 5.1 from stereo). And the reason these 5.1 headphones do not come cheap, is because they come with their own decoder. Your PC 5.1 headphones were less expensive because there is no decoder; the decoder is built into the pc sound card.
If you already have this, how does it work? See what your reciever box (decoder) reads of which type of signal it is outputting with your headphones (DD 5.1, Dolby Pro-Logic II, etc.), and let us know.