I can imagine that the Java objects that are annotated for Hibernate are coming from a database schema that initially created with regular tools like a DB designer, isn't it ?
Else, the only solution we have is to let jBPM create the schema the first time he runs on the persistence DB and then generate the DDL from the automatically created tables and other DB objects.
For info, I am in the same situation where I want jBPM to persist objects in an Oracle DB but I also want it to not create the schema each time my application is started. So I configured the hbm2ddl.auto property to 'validate' once the schema was created.
I don't know if it is the recommended practice.
rgds