You cannot affect these properties from your Java program. You can even overwrite the corresponding system properties, but you'll only succeed in confusing anyone who reads those properties.

If you need to load classes dynamically, then use a ClassLoader. Your custom class-loader can load classes from anywhere you like. If you want to alter the path so that you can load libraries dynamically, then override the findLibrary() method in your custom class-loader. There must be several FAQ entries that explain what to do.