http://hwellmann.blogspot.com/2011/03/jaxb-marshalling-with-custom-namespace.html
This shows how to do it.
Another:
http://www.systemmobile.com/?p=280
Key bits in case that link dies too:
the NamespacePrefixMapper class, found in the com.sun.xml.bind.marshaller package. The abstract class has one method to implement:
public abstract String getPreferredPrefix(
String namespaceUri,
String suggestion,
boolean requirePrefix);
then
Marshaller marshaller =
jaxbContext.createMarshaller();
marshaller.setProperty(”com.sun.xml.bind.namespacePrefixMapper”,
new MyNamespacePrefixMapper());
If you’re also using javax.xml.xpath.XPath, your NamespacePrefixMapper can also implement javax.xml.namespace.NamespaceContext, centralizing your namespace customization in a single class.