We had an issue the other day when journaling failed for a period of time and we needed to retrieve those emails and add back into the journal mailbox. To do this, you can use the Exchange 2013 New-MailboxSearch cmdlet to retrieve the emails and copy into the journal mailbox. Use the below cmdlets to copy all emails from 09/07/2015 to present to a mailbox called journal:
Get-Mailbox | New-MailboxSearch -MessageTypes Email -TargetMailbox journal
-StartDate 09/07/2015 -Name JournalExport
Get-MailboxSearch JournalExport | Start-MailboxSearch
The emails will be copied to a folder inside the export folder in the journal mailbox. You need to add this folder to the list of monitored folders that your archiving/journaling solution pulls emails from.