I've done it by exporting the XML, editing the XML from several courses together, and then re-importing it. You can dump all the materials from the html, problem, video, tabs, etc. folders from one course into the other, and then edit the course/course.xml files together. You may also need to merge some of the policies/whatever.json files together as well, and I'm not sure what will happen if your courses have different grading types.
I would definitely create a new course for the re-import, rather than trying to overwrite an existing course.