It is pretty easy to do this on-the-fly with information you provided. Please check sample yaml demonstrating how to do this. Here is build demonstrating this in work.
Sure in real life you do not need to download Test.xml as it will be created by your tests, and also you can choose to check-in ctest-to-junit.xsl instead of loading remote file every time.
Thanks for that Powershell magic Ilya. It’s working well for me here. It would be useful to have this information in the Appveyor docs perhaps (could also add info on googletest/gtest which makes junit compatible xml by default).