Hi Mario,
User custom fields should already be available - they should appear in the columns and filters pulldown menus as soon as you add a new one. They do appear in their own section though so it's possible you may not have spotted them - look for a section called 'User Profile' (below the user section).
Course custom fields do appear in the 'Course' report source (again in their own section called Course custom fields), but I'll need to add them to the course completion one.
Simon