How to add new java classes to the existing classes in jar file

I created new modules for software written in java. This software is run from jar file.My classes are extending the classes in this software. I want to write a plugin which adds new classes to the jar file. I know how to extract a jar file to the directory and add them to the jar file using java.util.jar package but as there are a lot of files to extract from the file, is it possible to add new classes without extracting the files to the folder, somehow directly.