You can do that, but you'll have to use e.g. the mod envelope or the mod wheel as a DC modulation source for the panning.
Using the mod env:
* Set mod env A,D,S,R to min,min,max,max - this makes it a quasi DC source that goes to max in 1ms during the attack and dies slowly towards 0 on release
* Press Morph, press Edit A, connect Mod Env to Voice Pan, set Amount to 100%
* Press Morph, press Edit B, connect Mod Env to Voice Pan, set Amount to -100%
You can also use the Wheel instead if you don't need it for other things:
* Press the Wheel assignment button to select "Matrix"
* Set the wheel to max (the set wheel value will be stored with the patch)
* Press Morph, press Edit A, connect Wheel to Voice Pan, set Amount to 100%
* Press Morph, press Edit B, connect Wheel to Voice Pan, set Amount to -100%
The FX will always process both outputs, so you'll have to use external FX for separate A and B (now left and right) processing.