Hello Uwe,
You can have any starting note number and compass for the Rank table, and up to 128 keys in a division/keyboard. You can also use any increement between the division/keyboard and the rank using the StopRank table. *However* the first note number on the division/keyboard is fixed at 36 (the intention was to minimise the number of settings) so I'm afraid you can't have a virtual keyboard starting at MIDI note number 28 in the CODM.
What I suppose you could do as a work-around is effectively to make the first virtual keyboard note behave as if it was note number 24 (i.e. have a -12 increment for all StopRanks) and then set a +12 key shift in Hauptwerk's MIDI input path. That wouldn't be ideal since it would need changing each time you switch to a different sample set.
A neater alternative would be to edit the generated organ definition file and change the Keyboard.KeyGen_MIDINoteNumberOfFirstKey setting from 36 to 24.
Best regards,
Martin.