I don't have a flight controller some there's some guess work below but it may help...
The normal and sport mode on the i6 TX have nothing to do with flight controller modes. They are associated with the programmable rates setting on the TX where normal is low rates and sport is high rates. Rates are a TX feature that reduce the maximum throw of the various controls surfaces, ail, elv and rud. On the i6 these are associated with particular channels so normal/sport modes only affect those channels
As I understand it, flight controller modes are particular programs that run on the flight controller that exhibit particular flight behaviour. They are typically selected with a particular servo position signal on a particular channel. On the i6 you might associated CH 5 (or CH 6) with the three position switch giving the ability to switch CH 5 between servo positions, -100, 0, +100. Presumably you can configure the flight controller to respond to CH 5 and switch between 3 different flight modes using this.
To select a particular flight controller mode you need to associated it with one of the positions of the the three position switch on CH 5 which will send either -100, 0 or +100
Possibly you may need an intermediate servo position to select the mode you want, say -50%. So the question is how to get the three position switch to send -50% rather than -100%
I don't have my i6 available to try it but probably you can use a mix to do this using the three position switch as the source and setting the -% it sends on CH 5
You might be able to use the pitch curve function since possibly you can set the the servo position for any particular position of the associated switch but since it requires you to use heli mode it may add some unnecessary complexity
Probably the mode you describe is one where the central region of the right stick motion say -10 to +10 of the ail direction and -10 to +10 of the elv direction are detected as indicating that level flight is desired and the flight controller automatically levels the multi rotor. Normally an airplane would just continue in the current direction if that stick is centred. My guess is that once the mode is selected then this behavior is automatic.
