Now you can set the reinforcements FGs to arrive when the Special Craft of the convoy is destroyed.
You may want to give them all the same cargo so the player can't identify the SC in-game.
Now have one of the convoy ships be the Special Craft of the FG, chosen at random. Make an FG containing a number of space freighters. Let's say you're attacking a six-ship Imperial convoy. I want to make a mission where ships arrive at a random time, for example to give the mission more replayability value since it can play out in different ways, or just to the player or keep the player on their toes, but I haven't been able to think of a way to actually implement this. One of the headaches I've had with XWA is that the editor doesn't do randomness very well.
If anyone wants to give it a shot and report your findings, I'd really appreciate it. Disclaimer: embarrassingly, I don't actually have XWA installed at the moment, so I can't test this out for myself.