Can I merge archived emails back into the mailbox?

We have a user that is leaving the company and would like his mailbox, including his archived emails, exported to a PST. This works fine with creating an Offline Copy and exporting his mailbox and the Offline Copy to PST's. However, we would like to merge the two PST's together. Is this possible?

I've noticed that when creating the Offline Copy, it actually puts the attachments back into the email, but not the body of the email.

Hope I've made sense here...

Mike.