In Session Horns the Velocity Switch uses Velocity
not aftertouch. In the Performance instrument for example, you can set velocity to control normal / Crescendo (FoPiCre). High velocity (strike the keys harder) causes a Sforzando (FoPiCre).
But I don't see any way to change the velocity switch with a midi CC or keyswitch. You might have to set up separate tracks for each velocity switch you want to use or use something like Transmidifier to map keyswitches to midi channels. For that to work you would load a separate instance of the single articulation instrument and set each to a different articulation and midi channel. Then set up keyswitches in transmidifier that map to the different channels. Not a quick fix but once you got it up and running it should work well.
One option might be to load a separate instance of the single articulation instrument into Kontakt for each articulation you want to use. Then go into instrument options (the gears) and set a different velocity range for each articulation. You'd want to control dynamics with CC11. I'm not sure if that would work.
eInstruments should have allowed the articulations to be automated with keyswitches.