How are the Migration Reports populated?
The Migration Reports are populated through a normal mailbox gathering (Default Gathering) from both organizations. The only additional thing that needs to be kept in mind is that two mailboxes form different organizations are included in this report when the data in the Custom Attribute 13 field is the same. This was done as the actual Migration products use default Custom Attribute 13 to put information to essentially tie the two mailboxes together.
Therefore, to get information in these reports, you need to use the Migration products to set Custom Attribute 13 on the given Mailboxes because MessageStats will only populate the report with data from custom attribute 13.