Good morning,
It is basically the same thing as in Hauptwerk 4. GO to Organ settings | Advanced audio routing | Rank routing to audio. On the right pane you will find the "Bass part of rank" tab where you will find the similar controls that were used in Hauptwerk 4 to route the bass to different channels (bus).
Note it won't be a "smooth" transition. At the split point the audio will be route from your regular speakers to the subwoofer. That' why you will probably want to include in your "bass buss channel" not only a subwoofer but also some satellite speakers so you can hear the higher harmonics of the notes played thru that channels/bus.
Your speakers and subwoofer crossover will take care of properly route the audio frequencies to the sub and satellite speakers.
Regards