OK, that may be the best option, instead of trying to hack something together.
For future reference, Realtek isnt windows, and the asio4all driver will completley replace it (while you are using it. everything goes back to normal once mixxx is closed)
You may want to try this: (on windows 7 32 bit at least, and i also have the realtek soundcard)
Open the control panel => go to "hardware and sound" => go to "sound" => right click and show disabled and disconnected devices, just so we know what we got => chances are the input is called microphone, open its properties => go to tab "advanced" => look at "default format", is there a 2 channel option? if so, you may be in luck.
then just select it in mixxx using asio4all.