In addtional to Mark's suggestions. There was once a post here about using the bpstart_notify and bpend_notify scripts to shutdown outlook on the client computers during backup.
(Look up the *notify scripts in the Nbu admin guide if you don't know about them)
I couldn't find that post anymore, but it goes something like this:
1. Backup schedule kicks off
2. Master server remotely triggers a script on client computers that gracefully shuts down Outlook.
3. Backup PST files successfully. (Because they are not locked)
4. After the backup, another script is triggered to restart Outlook.
If you decide to go through this route, you will have to work out the scripts yourself...
Be careful with the backup schedule so people won't get their Outlook shut off when they're using it.
This method could be complicated and error prone.
Another way would be to simply tell the users to close their Outlook when they finish work, or find a way to enforce such a user policy; do this instead of trying to use elaborate methods to solve human/procedure problems.
As for DLO, as I said, it is a separated product, please refer to its admin guide:
http://www.symantec.com/business/support/index?page=content&id=DOC5268
You may also want to visit the DLO forum:
https://www-secure.symantec.com/connect/backup-and-recovery/forums/desktop-laptop-option