I think this would be a useful enhancement to EclipseLink's JPA support,
please log an enhancement request for it in Bugzilla.
In general, EclipseLink supports adding class dynamically through its'
native persistence API, but not directly for the JPA API, as JPA does not
support this.
However it should be possible to add JPA Entity classes on the fly. You
can access the EclipseLink ServerSession from the EntityManagerFactoryImpl
(getServerSession()), and use its' addDescriptor(ClassDescriptor) or
addDescriptors() API to add EclipseLink ClassDescriptors. You will need
to build the ClassDescriptors meta-data objects directly yourself (or use
the Mapping Workbench to create them), as loading from JPA annotations or
orm.xml would be more difficult.
There is also the issue of weaving the classes. Since they may have
already been loaded, they will not be able to be weaved, so will not
support lazy ToOne's or change-tracking unless you statically weave them,
or use ValueHolders directly.