You might be better off creating a separate group for each category. You will still have to go in and link that group to each lesson, but once it is done, then whenever you add a student to the Matematik group, they will be added to all the pertinent lessons. You can also use that to keep students out of lessons they do not need to be in. Here is more information on groups/groupings http://docs.moodle.org/23/en/Groups
When you are saying category are you talking about a gradebook category or just the title in the topic section?