Would the "Pan/Expand" process be able to do what you want? From reading the help file, it seems that you can fade a channel from left to right, or right to left. It also says you can "perform right and left channel panning effects by mixing the two channels and then changing the volume between channels", and you control it with envelopes.
I think I understand what you're trying to do, but I'm not sure from the process description whether the Pan/Expand will do it or not. I know n-Tracks allows you to easily set up panning envelopes, just like volume envelopes. It doesn't seem as straightforward in Soundforge.