I had a look over that issue request you linked and saw your? comments.
I hadn’t replied to this earlier because honestly I don’t know much about xunit, so I was hoping someone more knowledgable would reply, and no point me cluttering up the thread.
It seems the feature you want is not there (by the request to add it), that issue is only 22 days old, so there is a good chance none of the RF maintainers have had a chance to look at it, it is an open source project after all so you could add the feature if you have the ability and issue a pull request.
If you don’t feel you have the ability to update the RF code, or you need a solution quickly, rather than post processing the output.xml file as discussed in that issue, another option would be to create a listener file that could generate your xunit xml for you. The Listener interface functions end_suite and end_test probably have everything you need and so you probably only need to write a listener function for each of those, and maybe one for start_suite that simply creates your xunit xml file with an initial blank template.
I hope this was helpful,