This can’t be done without modifying the engine. Classes need to be marked as exported (prefixed with THEMODULENAME_API) for you to be able to use them across modules. If the class is not (as in this case), the only option is to modify the engine code with the prefix.
If you do this, you should ideally then submit a pull request to Epic, they are generally pretty willing to accept these kind of PRs into the engine.