There is an example of this in the docs
http://www.hibernate.org/hib_docs/v3/re ... /#d0e10435
See 18.1.1. Specifying XML and class mapping together
This allows you to have both POJO and DOM4J XML mappings for the same entity.
As for merging the metadata for a .hbm.xml file and the anotations - I don't know if the hibernate Configuration supports that now - but it could certainly be written - are you volounteering ?
There is also sample code there about how to start a EntityMode.DOM4J session to extract the data in XML.
You can use a normal POJO session to create the POJO data in the database, and use a DOM4J session to extract it in xml.