So you want the file
<UpdatedFiles>
<File FileName="PNet.GlobalConstants.dll" From="1.6.3.73" To="1.6.2.16"/>
</UpdatedFiles>
to be updated like this?
<UpdatedFiles>
<File FileName="PNet.GlobalConstants.dll" From="1.6.3.73" To="1.6.2.16"/>
<File FileName="somefilename.ext" />
</UpdatedFiles>
The Write XML action doesn't support creating a new element and attribute like that. You could write some script code in a Run Script action to do the specific query and update you want to do.
http://www.google.com/search?q=msxml+vbscript