Yes, the MP3 and Kemper could be recorded in real-time into the Steinberg.
From the Manual:
The RETURN and ALTERNATIVE input can be used as the stereo loop returns for external stereophonic equipment by selecting “Loop Stereo” in the EFFECTS section (X or MOD). Both the RETURN and the ALTERNATIVE input can also be used as aux- iliary inputs. You could, for instance, use this feature to mix in music from your Mp3 player to the sound of the PROFILER to play along. You will find the corresponding mix controls in the Output Stage.
Awesome, thanks. I remember reading that at one point but it's great to know that both will go out to the Steinberg in one 'package.' Saves me from having to merge the files in the DAW.