You can add subjects to your course by simply dragging them to your course map.
If you are the teacher of the course, you should see the 'Edit' button on your course overview on the top right.
If you did this correctly, a side-pane opens on the left.
Here you see all the repositories you have access to.
The repositories are divided into three sections:
Your (personal) repositories: these are the repos you have created or you have been directly invited to.
Organisation repositories
Community repositories.
If you click on the arrow in front of a repo, it will open the repo and you can see the subjects it contains.
If you find a subject you want to add to your course, you can simply drag it to your course.
Here's how: click the three lines in front of the subject and hold down the mouse. Now drag the subject to the right place in your course. That's it!
You can also search all your repositories at once. Just type in part of the subject name in the search bar and hit enter. All the subjects that match the term will be displayed. Drag them to your course to add them.
If a subject is already in your course, you will see a checkmark and the font color is grey.