In general, if you want to control the volume of a sound, it's simplest to do this with a knob/fader that sends midi.
If you are wanting to control the volume of Sunday Keys Section 5 which isn't automatically mapped on the nanoKontrol2, you can re-map a different knob/fader like the furthest right knob or fader for that purpose. You might also choose to use the TouchOSC App to control it as well as the Tonic Pad Player functions.
In short, you'll want to go into Layout Mode, click on the control in the workspace that you want to re-assign, for example the furthest knob and then choose Type: Unassigned. From here, click on Section 5 volume fader, click "Assign" then move the physical knob and deselect "Assign". Now that knob should control it without any other mappings.
Watch the video below to learn more about how to map your controller for these purposes in Video 9: Connecting Any Knob, Button, Fader, or Controller to Sunday Keys on the Sunday Keys Resource Page.