Conventionally when you import a Student List via the 'Import' sub-section in the 'Students' tab of the LabTutor Server, you may only initially choose to assign the imported students to a single Restricted Course.
You can quickly add the existing students to another restricted course by simply re-importing the Student List (.xls / .csv file) containing their details, only choosing the next Restricted Course you wish to add them to during the import. Since the students' details already exist (i.e. Username and UserID), their details will just be updated to add them to the selected restricted course.
This can then be repeated as many times as desired, to quickly add students to other restricted courses.