while I was at it, I also got the pack running you have mentioned...
actually it is quite easy once you know what you are doing;
you can open the classfiles with notepad and so you can sort of "read" what this classfiles is about;
in the case of the Kamikawa Maru, there was only one classfile inside that showed reference to that ship, while the others referenced ship I have never seen released for Il-2;
the bigger classfile (the one I told you to copy over and replace the one thats inside ships.classes folder) is referencing all ships, the creator of the mod did up to the time he released the Kamikawa Maru; it was bigger than the one originally in the ships.classes folder, so it was logic to copy and replace it with the file that came with the Kamikawa Maru
of course backing up everything beforehand helps a lot
in the case for the pack you have mentioned, try adding it and leave out the classfile you have copied over to ships.classes already
should work - if not, report back here and I try to get you going