It seems the Tune key is missing from the selection list in the config screen for the midi keys.
I found it in quisk_settings.json (the directory where quisk is started from). In the section "MidiNoteDict" i entered the Note ID of the rotary encoder of my device and set the action to "Tune +0" which gives a minimum stepsize of 10Hz.
What would be a great feature is if a pushbutton could be used to select a certain slider and then a defined encoder could be used to control this. Then one could for example select RIT and control the RIT and then select volume and control volume from the same encoder. Currently the logic seems not to allow for that. Anyhow its already great. So many thanks!