Very possible.

Simply set your rudder 2nd rate to be on a stick switch.

Set rate 1 at your high rates (say 100%).  Then set rate 2 to be your low rates (say 50%).  Go to the SW menu and set your Rudder 2nd rate to be on SS1 (Stick Switch 1).  (The Stick switches are after you've gone through all the numbers...ie. it's after Switch 34)  Then go to the Stick Switch menu (it's one below the switch menu) and go to Stick Switch 1.  Set the stick to be THR, and then move the stick to the position you want to activate the rate change and push the + key. 

Make sure the setting right below that is set to H (that means that the switch is ON when the stick is High, rather than vice versa.)

You're done.

To confirm, just hold your rudder at full deflection as you increase the throttle stick (with the motor off) and you should see the rudder change at the point where you set the switch.

Let me know if this helps...it's a little convoluted as I read this...lol