Convoys, ships, planes and all units are scripted in the SCR, LND and RND files in the Campaigns/Campaign folder. They have specific routes and dates where they appear and dissappear. Of course, you can add convoys or single ships, or airplanes to your liking using the SH3 MisEditor, but if I am right, there is limit to what you can add to those layers before it starts doing funny things. I am not 100% sure about it, but maybe someone will shed more light on this. Hope this little piece of info helps you.
