Yes it is! Just cut off any further (new) MIDI note-on/note-off input to any synth once you have the sound ongoing. That is what you do when you use the SUSTAINER module with the FREEZE function in MG3. First you send a Note-on message to a synth (instructing it to start sounding) and after that you presse the SUSTAINER pedal to make sure it won’t get any new information to stop that already ongoing sound.
You can use the MIDI Filter in GP5 to the same effect. Instead of using a channel for CC 64 in MG3 you can probably connect your Morningstar expression pedal directly to bypass a MIDI Filter in GP5. Set the filter to cut out everything (necessary), or at a minimum “note-on” and “note-off”.
But if you learn how to do this you can just as easy set up a expression pedal in GP5 itself connected to the other output of the Morningstar, and you don’t have to go the extra mile with the double chains MIDI OUTs in MG3.
But how you are going to get to the pairing of these two simple functionalities to each and every instrument you have in your rack independent of each other is beyond me.
You haven’t told us how you activate or select these in GP5 yet.
Link to @Vaultnaemsae video: