Hello phdcam,
The only way that you could extend a sample set's compass yourself would be create your own organ definition for it (usually from scratch) using the CODM, with whatever virtual specification you like. However, that would normally involve learning the CODM fully, and it's moderately technical and involved, I'm afraid.
In the case of St. Anne's specifically, the task is made rather easier for you because a CODM organ definition for St. Anne's is included and installed with Hauptwerk anyway as an example:
[Hauptwerk installation folder]/HauptwerkUserData/CustomOrganDefinitions/ExampleCustomOrgan6-StAnnes-WithAllFrills.CustomOrgan_Hauptwerk_xml
Hence you could make a copy of that and then edit it to taste. You would still need to read the CODM's user guide to familiarise yourself with its basic use, but the following is a summary of the attributes that you would need to change (for the relevant division and ranks):
- Division.NumberOfKeys
- Rank.NumberOfPipes
- StopRank.NumberOfMappedDivisionKeys
Doing so wouldn't be ideal for realism/quality, since the highest pipe sample in any given rank would then simply be re-pitched in real-time for the additional notes (whereas a real pipe would have different attack/decay speeds and resonant frequencies, for example). However, if extending by just a couple of notes then the results would probably be considered acceptable.
Other alternatives might be to try contacting the maker of any given relevant sample set to see whether he/she has, or might consider making in the future, a version of the sample set that's extended accordingly, or to contact Jake (forum user 'subbass32') to see whether he might be prepared to create a customised version of the relevant organ definition for you, on a consultancy basis: viewtopic.php?f=6&t=15558&start=60#p119505 .
I hope that helps.