Adding node(s) to XML process variable

I have a process with an XML variable, i.e.

var myXML = <package>

                        <assets>

                            <foo name="A">....</foo>

                            <foo name="B">....</foo>

                            <foo name="C">....</foo>

                         </assets>

                  </package>

and I need to add an additional subtree <foo name="D">...</foo> to this variable under /package/assets/.

I am trying to use SetValue but cannot get it to work.

What's the best way to do this?