I've never tried this, but I wonder if you import everything but the calendar, then copy the calendar into the mailbox, if that would work.
https://www.slipstick.com/outlook/calendar/move-meetings-losing-ability-send-update/
The other issue is the legacyExchangeDN. If the mailboxes were moved using a hybrid scenario, then it will be preserved. If they were created clean in ExO, then you would need to add the old LegacyExchangeDN of each mailbox as a X500 address to the new mailboxes in Exo