Rather than using Greenfoot, I've just imported the Greenfoot library into IntelliJ so I can code there externally. If I wanted to import that Java code from IntelliJ back into Greenfoot to test, it would require the construction of a project.greenfoot file. How are those built by Greenfoot, or how would I put one together myself so that the code would run in Greenfoot?
The world's worst IDE will automatically pick up .java files in the project directory on open. So the easiest way to dump a bunch of Java files into a Greenfoot project is to make a new project, close Greenfoot, delete all the files except project.greenfoot (and the images and sounds directory), copy in your .java files, and re-open Greenfoot.
Incidentally, you don't have to take your code back into Greenfoot to test it. We have instructions on how to run Greenfoot projects within NetBeans, which I think shouldn't be too hard to translate across to IntelliJ. That would allow you to develop and run Greenfoot projects from IntelliJ without ever opening the Greenfoot IDE.