Relevant Product: Mail Archiver
You will be able to use the Batch Importer feature to import content from existing users mailboxes. If you are also using a Personal Archive Mailbox(es), you need to export that archive mailbox to a PST using Exchange Management Shell. You can them import the PST via the Mail Archiver Batch Importer.
To export the archive mailbox you can use the following shell command:
An example of this would be:
For more information on the New-MailboxExportRequest cmdlet, see: http://technet.microsoft.com/en-us/library/ff607299(v=exchg.141).aspx
To set the script to export to PST for each mailbox, you could use:
You will then need to obtain scanpst.exe from a machine with Office installed (drive:\Program Files\Microsoft Office\OFFICE12) and run scanpst.exe against the PST files you will have created.
Then use the following option in the Batch Importer to import the PST file: