I am new to Office 365, but it seems fairly easy to be able to convert a licensed mailbox into a shared mailbox in the event of an employee exit. You can then share out the mailbox to other staff. Easy.
But, if the employee who left is a longer term employee who has an online archive (we set ours for 2 years to move from main to archive) you have a problem. If you convert to a shared mailbox, you lose the archive.
An alternate suggestion would be to convert the mailbox into an "inactive mailbox" which is an enterprise level option relating to litigation hold. This allows you to retain the information forever - and release the license. But, it cannot be accessed except via ediscovery tools and would have to be converted back into a mailbox for it to be shareable.
So, after that long winded intro, what I think I need is a procedure to consolidate an archived mailbox back into it's main mailbox so that the fully consolidated main mailbox can then be converted into a shared mailbox. Exporting the archive to a PST probably is not quick or efficient. And then, what do you do when you have the PST? Import it into the shared mailbox? That will take longer than the creation of the pst.