That's just swell.

Greetings,
I am a ignoramus of computers, hence my need to ask many dumb questions, like this one probably is: Can a swell pedal be added to a Hauptwerk instrument that does not have a swell, such as the Zwolle, Freiburg, etc? i.e. can I add a volume control that would (in a pinch) give the effect of a swell ?

You can add a volume control by connecting to HWs volume adjustment which would act just like the volume control on any amplifier but the effect won't model a real organ's enclosure opening and closing. The muffling effect of closing shutters on a real organ will be missing but it will certainly reduce the sound level.
Try playing a radio and reduce the volume. Then leave the volume set, exit the room and close the door. Totally different effect although the volume is reduced in both cases.

Thank you for that information.