Sono riuscito a trovare una soluzione:
select XMLElement("Changes",
XMLElement("Description", 'sometext'),
XMLElement("Notes", 'sometext'),
XMLElement("FulfillmentDate", 'Some Date'),
XMLAgg(XML_CANDIDATE) ).GETSTRINGVAL() RESULTS
from
(
select XMLAGG(
XMLElement("UserChange",
XMLElement("Operation", 'sometext'),
XMLElement("User", 'sometext'),
XMLElement("BusinessSource", 'sometext'),
XMLElement("ApplicationRole", 'sometext'))) XML_CANDIDATE
from
table);