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:
New-MailboxExportRequest -Mailbox ##mailboxname## -IsArchive -FilePath ##UNC address to pst file##

An example of this would beNew-MailboxExportRequest -Mailbox jjohnson -FilePath “d:\archives\jjonhson\archive2010.pst” –IsArchive

For more information on the New-MailboxExportRequest cmdlet, see: http://technet.microsoft.com/en-us/library/ff607299(v=exchg.141).aspx

If you find that the cmdlet is not available, please see: http://www.msexchangeblog.nl/2010/09/23/missing-commandlets-in-exchange-2010-sp1/

To set the script to export to PST for each mailbox, you could use:

foreaforeach ($i in (Get-Mailbox)){ New-MailboxExportRequest -Mailbox $i -FilePath "\\server\mailboxexports\$($i.Alias).pst" -IsArchive}
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:

How To Mail Archiver