If you do not mind buying online (if you've never done it, try it, its a lot easier and more secure than you might think), I would recommend this GWC USB 5.1 Adapter
. Considering it has the 5.1 Output capability and a standard mic in, I would assume it will have that basic feature (stereo mix) which is built in to most integrated sound and sound cards today (all Realtek and C-Media devices have stereo mix capability).
I would do some research on that product, and if you do not mind, purchase it and see. If it does not, then you can return it back to Newegg for a full refund.
Good luck, Qwertywasd!