Nearly 3 years after the original request, using Studio One 6.6, I have a very similar issue.
I'm trying to add a Generic midi controller, which has 4 nice knobs of the type high-res endless encoders. In Bitwig Studio, which I've been migrating away from, this controller correspond to a setting called Relative Bin Offset, and works nicely.
For Studio One, in the External Devices -> MIDI Learn, I see the Midi controllers for the knobs, but I cannot seem to find any way to specify that the "Knob" type is relative, not absolute, values.
Tech Support just told me, that it's not currently supported. Would be very useful though :)