Restoring On My Mac mailboxes from backup

I recently had to erase my iMac hard drive and reinstall everything. I have pretty much gotten everything back except for my wife's Outlook messages, my Firefox bookmarks and the local, On My Mac, mailboxes in Mail.

I have located the .mbox files and I have verified that the messages are still inside those files. I have copied them into my user Library but they still do not show up when I run Mail. What part of the secret sauce am I missing? I have searched extensively on the web to no avail. Everything just goes in circles.