My Approach
The way I do it is assign a soft knob to a 'Control #' such at Control 9 or 15 or whatever then map that in the matrix. Your DAW will then send CC#9 or 15 or whatever to the soft knob. IIRC correctly, the several oddball values that the knobs can be assigned to such as breath, velocity, pressure, etc. are relatively standard CC values that can be looked up online. You're not actually implying you're going to use, for example, a breath controller, but agreeing to use that CC value which might have the added advantage of being called the same thing in another tool. In the case of pressure, your keyboard may send aftertouch (channel pressure) and would likely do so on the standard MIDI CC #. In this case, you wouldn't have to do anything extra to make it all work.
-CCQ-
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