Piece of cake
Go to channel setting app
Page over
Assign steering servo to an aux channel
Assign to a trim switch. ( I use T5)
Just over the throttle trim
Now you can adjust steering without effecting rudder

Go to program mixing app
Mix to rudder to aux channel use the gear switch (C) to enable/disable mix
Set up percentage of mix on position1, leave position "0" at 0 %
You are done.

Shepp