Because you say the XML is read-only, I think your first issue is that
you're trying to make the change in onBeforeChange. All XML is read-only
there; you should use onAfterChange if you want to set data after a user
modification.

Here's some sample code to help you out. You'll have to update the node
names and namespaces for your specific solution.

// Takes HTML from a plain text field and inserts it as XHTML into a Rich
Text field.
// HTML could come from another source. It must be valid XML, and this is
not verified.
var textfield = null;
var htmlfield = null;
textfield = XDocument.DOM.getElementsByTagName("myNode:node1")[0];
htmlfield = XDocument.DOM.getElementsByTagName("myNode:node2")[0];
if (textfield.text != "")
{
   var xdocFrag = new ActiveXObject("Msxml2.DOMDocument.5.0");
   var ns = "xmlns:xhtml=\"http://www.w3.org/1999/xhtml\"
xmlns:myNode=\"http://schemas.microsoft.com/office/xdocs/2003/myXSD\""
xdocFrag.async = false;
   xdocFrag.validateOnParse = false;
   xdocFrag.loadXML("" + textfield.text + "");
   var newChild = xdocFrag.documentElement.firstChild;
   htmlfield.parentNode.replaceChild(xdocFrag.documentElement.firstChild,
htmlfield);
}

Thanks,
Brian