Marshalling List of Object without affecting existing list

Marshalling List of Object without affecting existing list

I need to convert a list of object from java into xml files using JAXB.
example:
> <?xml version="1.0"?>
> <order_list>
> <order id = "1"/>
> <order id = "2"/> </order_list>
without affecting it list of order in xml file , method to add one order
id at a time to the existing XML in the above format.
the new order is always added at the last child node in the XML.