That functionality is not currently possible. It is being tracked as enhancement request ODMEM-I-72: when migrating email mailboxes (and M365 Groups), also migrate aliases
We had made a decision earlier not to migrate aliases because of situations such as, for example:
user1@source.com has alias1@source.com and migrated to target but user2@target.com has alias1@target.com thus user1@target.com can't be given the alias1@target.com email address.
We are reconsidering that prior decision because:
--The target could be brand new and so there are no aliases at all in target.
--Even when aliases do exist in target they would mostly be unique.
--We could have a way to handle conflicts, such as to not migrate that particular alias and flag to user that that alias did not get migrated and why.
