@cheikkila There are two ways that you can have the student's role changed from student to withdrawn_student with your SIS imports, but which is the better solution depends on if you include ALL enrollments in the imported CSV or just the changed ones (a "pre-diffed" CSV). 

If you are including all enrollments in the enrollments.csv, then you can use Diffing Mode with a diffing_drop_status set to deleted. Then the active student roles would be "missing" from the data and result in the student enrollment being deleted and the withdrawn_student role getting added.

If you are only imported changes to the enrollments, then you would want to list the withdrawn student's enrollment twice in the CSV:  once with a role of student and status of deleted, and once with the role of withdrawn_student and a status of active.

You may want to consider an alternative to the custom role as well:  you could set the status for enrollments that are dropped during the drop period to completed and those dropped later as inactive (or vise versa) as both are still accessible in the gradebook for instructors to view, but inaccessible to students (the option to prevent students from accessing the course after it is completed would need to be enabled to "hide" the course from completed students otherwise students would only have read-only access to the course).