This conversion seems to be mostly related to the transfer of e-mail data from one computer to another, especially if each machine is using different program for e-mail management. You can find various 3rd party tools designed to help you with the transfer process.
Convert Mail mailbox to Outlook for Mac e-mail database.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert mbox to olm on OS X
Microsoft Outlook for Mac is e-mail client and personal information manager distributed with Microsoft Office for Mac. Outlook for Mac contains email, calendar, address book, task list, note list managers in one application. Microsoft Outlook for Mac is a successor of Microsoft Entourage for Mac.
Convert from mbox file
to olm file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.