Do you mean an archive as in a PST file on a user's hard drive somewhere, or an archive physically accessible in O365? If the archive is available via standard username/password login and you have an accessible hostname for it you should be able to use Settings>Connectivity>Mailbox Migration to import that data. If the data you are looking to import is in a PST file these can be imported by syncing a mailbox to Outlook via MAPI or similar, opening the PST archive, then moving/copying that mail data to an appropriate folder in the synced account. Please let me know if you have any questions and I'll be happy to point you in the right direction.
Kyle Kerst
Lead Internal Network/System Administrator
SmarterTools Inc.
smartertools.com