[CCIP]

No, it is not possible to edit save games in this way. It is possible to edit campaign layers to make certain ships appear at exact times in exact places, but it is very time-consuming and complicated (i.e. you'd have to add every single ship by hand, more or less, into the scripted traffic layer, then send that whole traffic layer to the other person involved). Editing campaign files is also not possible DURING a patrol, or at least highly inadvisable. It will likely make saved games crash, unless the game is saved in port between patrols.

In other words, this would be very impractical (you'd be spending more time editing than playing), and also remove all element of surprise to at least one of the people involved even if actually done.

I did do this on a limited level back towards the end of WAW 1, when we had a few events/ships/resupply points that were merged into the game's scripted traffic layer and given to people BEFORE the start of a patrol. However that wasn't possible for routine sorts of operations like convoy hunting, which mostly come from the game's random traffic layer, the whole point of which is to make the game's traffic dynamic or at least unpredictable.