As far as I know you can only get and set XML values using ST. You are trying to add new nodes.
It looks to me like you would have to use XSLT or some scripting language to process the actual .xml file throwing the output back at the browser. Both Python and Perl have reasonable libraries for both SAX and DOM.
However, I have no experience of testing XML with ST.
[This message has been edited by johntest (edited 12-13-2002).]
We were also facing same problem here. Workaround we found for that is as we do for any text file:
1. read each lines of the XML file to a string
2. search for the pattern in the string using "MatchStr" function
3. write the line to a new XML file
4. If the string matches write your <def>HELP</def> string line to the new file
5. Delete your Original File
6. Rename your new file as the original one.