You can…but it’s kind of weird. In the kmix editor you can set USB input on channels to “POST” which sends it to the computer after it runs through the faders and effects.
I’ve been having issues with it on Windows…I installed VoiceMeeter to route audio between applications. It mostly works, except I can’t get any sound from KMix unless I set all the usb inputs to “POST” which then mixes everything down to stereo. That’s not necessarily terrible…I can work with it, but I’m a little surprised. It may actually be an issue with VoiceMeeter…as it looks like it assumes all hardware inputs are stereo instead of multi-channel. It also doesn’t give me an option to choose ASIO as the driver for a hardware input.
Anyway…my issue here isn’t actually with kmix as I write this out, it’s about Voicemeeter and audio routing in windows. Possibly. Any chance you’ve messed around with Voicemeeter & Kmix?