You can use the "Compile To Library" option in the Project menu in the B4J IDE. This will compile your project (except for the Main module and other modules labeled #ExcludeFromLibrary) into a library with an .xml and a .jar file. If my recollection serves me, you will find those files in your Additional Libraries folder.
The .jar file can be used as a library in a Java project but you will most likely have to reference jCore.jar in your Java project and possibly other .jar files, as well.