Once I tried using sh3 subs in sh5 to see if adding subs was as easy as that. It sure wasn't
I moved everything properly .. crossed my t's and dotted my i's, I was able to add them into a mission as a player unit with the mission editor, and was able to select them when your in game choosing what sub to take into a mission.
But once the mission fires up with a non sh5 sub selected CTD .. nothing I could try would get it to work. Here's hoping others have better luck.