Anybody can tell me if there is a reason why "/opt/zimbra/zmhsm --start" is not included in the zimbra crontab?

I understand well that the settings you are doing in the administration UI is for the ttl of an object on a storage volume. But as we are currently migrating our users, I would like to have a setting for "Message age treshold" of a very short time (splitting imported/migrated documents based on there header information would even be better).

1. Do I have to execute HSM always manually?
2. Is zmhsm already included in some of the scheduled cleanup tasks?
3. Does including "zmhsm" in the zimbra crontab (every 6 hours) do any harm except higher server load during this time?