Hi Rebecca
It is only possible to add standard tags to a course and these appear site wide.
Once a tag is added to a course then you can configure the course catalogue to display the tags in the course catalogue filters
It sounds like you are wanting a personalised course catalogue - this is more of an idea for Engage which provides persoanlised content and will be able to connect to Totara Learn to provide content based on the user's tags (Topics)
Regards