Hello Thomas,
[Topic moved here, since only the CODM can automatically 'extend' ranks.]
If you specify a CODM ODF's rank to have a compass upwards beyond that of the samples then Hauptwerk will re-pitch the highest sample for each of those additional pipes. Conversely, it will re-pitch the lowest sample if extending downwards.
Since re-pitching samples to higher/lower notes will inherently adjust their attacks/decays (as well as frequency content), you can usually get better results by making the additional samples yourself in an audio editor (e.g. by re-pitching them manually, then applying filters and/or envelopes to them).