I do recommend this URL, with official API and explanation about how to assembling and transforming XML objects.
You have the prependChild() method or the appendChild() method to add a property to the beginning or end of an XML object’s list of properties. Also the insertChildBefore() method or the insertChildAfter() method to add a property before or after a specified property.
You can also use curly brace operators ( { and } ) to pass data by reference (from other variables) when constructing XML objects.
A quick solution (not telling you that is the best) for your answer:
var xml:XML = <imglist><url>data</url><title>title</title></imglist>;
var newXML:XML = <imglist><image>{xml.url}{xml.title}</image></imglist>
trace(newXML);