Using Import/Export-Mailbox on Exchange 2010

By default you cannot see the cmdlets import-mailbox either export-mailbox. To enable the Exchange Management Shell to use these cmdlets you need to run this command to allow a specific user:

New-ManagementRoleAssignment –Role “Mailbox Import Export” –User “Rodrigo”

or you can enable a group:

New-ManagementRoleAssignment –Role “Mailbox Import Export” –Group “MailboxAccessGroup”

After running one of these cmdlets restart the Exchange Management Shell to use the "hidden" cmdlets.

Source:
http://www.howexchangeworks.com/2009/08/where-is-exportimport-cmdlets-in.html

Related sites:
Export-Mailbox
http://technet.microsoft.com/en-us/library/aa998579.aspx

Import-Mailbox
http://technet.microsoft.com/en-us/library/bb629586.aspx