The X1800 can do this if you set the input selection to DVS. Make sure that in utilities that under ‘5. DVS Setup’ all channels are set to their proper line/phono. After that, just make sure you’re receiving/sending on the same stereo pairs on your computer for each channel.
DVS mode sends the mixer input to the computer and then sends the computer audio back out to the mixer channel. Typically this is for sending vinyl time code and piping back the processed track to the mixer.
In your case, it’s using your computer as an insert. If you do anything nifty, you should totally post it to the forum. I’d like to see what you’re doing.