I am searching for the same solution, do you finally find it?
I have tried to create it manually, but the compiler doesn't work and it is not able to access the additional fields. There is so limited resources or article talking about this.
You might want to post in the JDev forum for JDeveloper specific info. The EclipseLink wiki has steps for generating the model with ant or Eclipse: http://wiki.eclipse.org/UserGuide/JPA/Using_the_Canonical_Model_Generator_(ELUG)