Teachers can not add site-cohorts to courses. The can add category-cohorts to courses.
I have created a role Cohort enroller, see : http://docs.moodle.org/26/en/Cohort_enroller
However, this role can once again only enrole category-cohorts, and I want them to have the ability to add site-cohorts to courses as well.
I think this option should be made possible.