I'm not UD, but the way I do it is run a line from the Alt Output of my Mackie 1202 to the line input on the keyboard.
Then you can assign the mic, or several mics if you wish, to that output. By pressing the Mute button, it takes the channel out of the main mix and assigns it to the Alt output.
The same thing can be done using an effects send or monitor send. Just be sure the channel on the mixer is muted or the volume turned down.
DonM