The game is treating the DLC as built in content. I don't have the contz pack, but if they are individual container, you would have to clone them, reskin them and then import and commit them into Trainz. Then they may throw up errors which have to be fixed and even then I don't know if you have to add the kuids to a queue of something.
You might be better off just skinning a standalone container and sprinkling those about.
I am a little confused though, I thought shipping containers were international?