Due to HTML5 nature, that is something hard to imagine.
Writing data on a hard drive is something generally not allowed by browsers (except using the local storage plugin allowing a specific hard drive space in the browser itself).
And depending on where your xml file is located (hosted on an online server for example) that's even something not dependent on the browser itself.
It would require a server-side script made to edit the xml file and accepting/expecting commands to do so (commands possibly passed through AJAX).
So in the end a Construct 2 plugin designed to write to xml is unlikely as long as tried to be used in a web exported game.