You do have a "mixer" built in the Kemper. I run a single cable from the mixer monitor send for me into the Aux stereo inputs on the Kemper, L + R. Then take the headphone out on front and go right into my in-ear system. With the correct output settings, you get a mix of the IEM input and your guitar (in my case, bass) right out the headphone jack. Hit the Output button scroll to a particular screen and you have the ability to change the level of that aux input with a knob. It works perfect. WHen I get to a gig, we do a soundcheck, and all I have to do is turn this knob to get a new balance of my bass and everyone else. It changes slightly per room I find. Then I'm good. If I'm playing along and want a touch more of my bass, I just go in there and turn down everyone else a few clicks, and I'm all set.
I wanted to purposefully not use a separate mixer as I wanted my whole setup to fit in a 4 space rack. But I used a cheapo Rolls box to do the same thing. There's one in particular that combines a stereo input for your IEM feed, a mono/stereo guitar input, and even a xlr mic input with passthrough. So that would accomplish the same thing without using the Aux inputs on a Kemper. The Stage can do it, but you don't have an Aux input, so you can use the Return inputs instead.