At this time, only Code.org-hosted tutorials and content are assignable to sections.
Many tutorials listed on hourofcode.com/learn are hosted and managed by other organizations on their respective websites - we just promote them. Students can still access these other courses through hourofcode.com/learn, without being assigned that course from your dashboard.
Any course listed on studio.code.org is assignable to sections. You can also use filtering options on hourofcode.com/learn to only show Code.org-hosted activities.