So your question is NOT specific to FFmpegSource2. You want to know how you can open Avisynth scripts in the aformentioned applications.
I think this can not be answered in general, but at least for Premiere Pro a plug-in exists:
http://urchin.earth.li/~tomford/avisynth/
See also:
http://forum.doom9.org/showthread.php?t=47194