Here is what I think will do what you want.
1. For the servo that controls the nose-wheel... reprogram vis X-bus the servo to respond to
an Aux channel. For example AUX14.
2. Under SYSTEM -> CHANNEL SETTING.
set whatever knob, or slider, etc you want to control
I did it for INPUT 'H' which is for the knob on right side of the
'JR PROPO' label on the transmitter.
Test to make sure this knob moves the servo. It should
by default move back and forth full throw
3. Go to D/R EXP page and set the Dual Rate for AUX14 to
about 25%. The knob will now only move the AUX14 servo
a little about neutral.
If you use flight modes set this same dual rate for all flight modes
(this is done by selecting the blue color switch on the D/R EXP page
and select POS0 for each flight mode)
4. Next go to the 'Program Mix' page.
You need to mix 'RUDDER' to CH14
The end result is that moving the rudder stick will move the rudder servo
and the nose-wheel servo. The knob H will control a small trim
of the nose-wheel servo. The full-travel amount of trim can be varied
by changing the 25% value mentioned in step 3 above.
I think this is what you were trying to do ???