I think this is an artifact of the type checker. It will run on a single file before any of the lines in the file are run. So, if you're trying to use a class that's only being imported with a Machine.add declaration, that declaration is not going to run before the type checker gets to the line where you use it. But, if a file has two Machine.add declarations, then the type isn't used in that file, so the type checker doesn't complain, then at runtime the first .add is run, followed by the second.
I guess a Machine.import would need to compile and run the file during compile time, which might be non-trivial because I'm not sure that the compiler can be gracefully interrupted. Maybe the "import" keyword is the way to go. This might be straightforward to do by adding a few rules to the grammar and making import be a reserved word, and allow a number of import statements (only?) at the top of of a program.
I have definitely faced the same issue when I was working on utility classes.
~Jack