Did you take a look at ringnebula's post? HSM allows you to move all messages older then x days to a separate volume. You could set this up and then most of the messages for these accounts would be moved over to the HSM store eventually.
However to match the functionality you are talking about with a .pst export, you should look into the following command:
Code:
zmmailbox -z -m user@domain.com getRestURL "//?fmt=tgz" > /tmp/backup.tgz
This will backup the entire account to an archive file, which you can then import again later if needed.