I am using the hbm2java to generate pojos from my hbm.xml files.
What I want to ask is, if I only want to add a new property in the hbm.xml is there any attribute for
hbm2java to indicate not to start the generation of the pojo from the scratch but only to update it?

I have the same question for the hbm2ddl that I use (from org.hibernate.tool.ant.HibernateToolTask).
Is there any attribute that helps to only update the schema and not to generate it from the start.

Thanking you in advance.

See http://www.hibernate.org/hib_docs/tools/reference/en/html/ant.html#d0e1204

This details the use and output of the various tools.
hbm2java -> No - you can setup an inhertance structure for your code.
hbm2ddl -> Yes - See the update option.