Use D output in 42099 (4X4 X-treme Off-Roader) set How I can use the D output for an additional motor in the 42099 set? Or how can I use a custom control skin?
At the time of writing this, it is not possible to use the Control+ hub and motors in any other configuration than the already existing preset ones. This situation is expected to change, but there are no promises regarding functionality or timeline from LEGO.