Use EVPM to set the policy on the mailbox to archive everything. There are instruction for Policy Manager in the Admin guide or Utilities pdf.
Here is an example:
;This Script will archive every item in the Mailbox
;If shortcuts are left over, that's because the item is already in the vault
;Start Script
DirectoryComputername =
SiteName =
Name = oneshotarchive
CreateShortcut = false
DeleteOriginal = true
UnreadMaiL = true
UseInactivityPeriod = true
InactivityUnits = days
InactivityPeriod = 0
DistinguishedName =
Name = mailboxroot
Suspended = false
Filtername = oneshotarchive
OverrideArchiveLocks = true
RetentionCategory = Business
;End Script