Create WorldServer return package from Trados Studio 2015

I would like to create a plugin that allows the user to create a return kit in SDL Trados Studio 2015 and upload to the server from the Editor View.

What is the best method to access this functionality from the API?

I tried of course the FileBasedProject.CreateReturnPackage Method

But, this creates a Studio Return Package. This method produces a return package that cannot be imported to WorldServer, it produces this error message: "The import attempt failed because the selected file "Testpackage-2016516-10h59m0s.sdlrpx" does not have a recognizable extension by WorldServer, or metadata file is missing from the contents of the selected file if the file is a zip archive."

What is needed is an *.wsxz package created from this *.sdlrpx. The wsxz is essentially a stripped down xliff in a zip file that can be imported by the WorldServer.

Thanks in advance.