may not be a possibility based on the original MIDI Implementation Specifications. however, i could be wrong though, much has changed since 1985.
not sure if it still holds true that the Volume (fader) runs on a different MIDI protocol all while the CC number for Volume is CC7. Again the confusion here is and still is that CC7 generally controls the VSTi Main output Volume, however on certain VSTi, CC7 controls some “other volume” specific to that VSTi.
in any case the CC121 integrates with Cubase Quick Controls. Open up the QC Panel within Cubase and activate the LEARN function. Assign the MIDI controller information to one of the 8 QC channels. From the CC121, activate the QC controls by pressing EQ TYPE and ALL BYPASS. the top 8 knobs will now be set to control QC instead of EQ. just rem to turn off the LEARN function for the QC Panel.