Resonator,
Yes, you can use the KPA as a mixer with some limitations. Run your L./R main outputs of your DR880 into the Return and Alternative Input ports. Enable the Stereo Loop in the X Stomp in Effects and use the Mix control to blend the KPA's amp and DR880 signals. In this setup, the Stereo Loop is ahead of Reverb, Mod, and Delay so they need to be set to whatever sounds good with the overall mix. You lose the ability to use the effects on just the guitar.
bd