There are no specific restrictions on writing code for subclassing. It is possible to subclass any entity, facade or process class without having to change the way that class is declared or used.
New subclasses of existing classes should be written in new source files. All new source files should be placed within the source subdirectory of the EJBServer\components\<custom> directory. Where <custom> is any new directory created under the components directory that conforms to the same directory structure as components\core. The generated class hierarchy will dictate the packaging of the new source files.