The xml plugin only reads xml, it doesn't write.
You can look here for the xml file format:
So basically the best way to append data to an xml file is to load it with ajax or with nwjs, store it in a variable, then just add text to the end.
As an example, if you have a file called "animal.xml" added to your project you can load it, append data to it, and finally download it with this:
start of layout
--- ajax: request animal.xml
global text xml=""
ajax: on loaded
--- set xml to ajax.lastdata
--- add "<fish>tuna</fish>" to xml
--- browser: invoke download of xml