Well I am not a expert, but here is how I would solve the issue.
I am under the impressiont that both Customer and Product are already CMP. So create a Getter/Setter Methods for this new field and do the JDBC inside of them and throw some custom exception if there is some type of DB error.
If this field is required make sure to add the Attribute to the ejbCreate method and if the setter method for this new field throws the custom exception roll back the transaction and throw a create exception.
But then again I am not a expert.