The short answer is: you can't. The long answer is: it's technically possible, but it would be a real pain. Greenfoot was never designed to be a standalone API, so trying to use it like that is not supported. If you really want to do it, export (share) from Greenfoot as an application, then look inside the JAR, pull out all the classes and files and try to build an Eclipse configuration with that. But I'm afraid you're on your own with that.
If you want to move on to Eclipse rather than use Greenfoot, I'd suggest you look for a suitable Java library to support game development. A quick google turned up LWJGL and JGame.