There might be a tricky chance using jack, if there is a jack compatible stream listener.
You can start Mixxx, connect the Aux inputs to the system capture.
After that you can reconnect the inputs to the jack stream listener using QJackCtrl.
This is a bit hard, since Mixxx is not a native Jack client Bug #1172299
In any case it would be better to handle the whole use case inside Mixxx.
There might be a tricky chance using jack, if there is a jack compatible stream listener.
You can start Mixxx, connect the Aux inputs to the system capture.
After that you can reconnect the inputs to the jack stream listener using QJackCtrl.
This is a bit hard, since Mixxx is not a native Jack client Bug #1172299
In any case it would be better to handle the whole use case inside Mixxx.