No, DiRT is an all or nothing venture.
COBRAS offers individual object selection if you prefer to go that route.
You can restore individual mailboxes (assuming the subscriber is already created) going directly to the PST files if you need to go that route. You can use Outlook or ExMErge manually to restore messages (this wont, of course, create the subscriber in Unity for you). All the messages are stored in .PST files with the alias of the user as the file name in the message backup folder where you did the DiRT backup.
