With the new DSM 6.1.1-15101 it’s FINALLY possible to schedule raid scrubbing, without modifying through shell commands.
Open Control panel -> Task Scheduler:
Choose Create -> Scheduled Task -> Raid Scrubbing:
Enter task name:
Choose the “Schedule” tab:
Leave it as you like, but monthly is preferred!
And your “Raid Scrubbing” task is enabled and ready to run: