Two carriers, and WLM itself is the first: the same IMAP trick that moves WLM to Gmail moves it to 365, since a 365 mailbox speaks IMAP too.

The self carry: add the 365 account inside WLM over IMAP, server outlook.office365.com with SSL, the account signing in with its 365 credentials, app password style access depending on the tenant's security posture. The 365 mailbox appears in WLM's folder pane and the storage folders drag into it, uploading server side batch by batch, a few hundred messages per drag on that aging client. Everything arrives as real 365 mail, visible in OWA and the new Outlook immediately and the stalwarts can even verify their own history landing as it uploads. Tenants enforcing modern authentication strictly may refuse WLM's older sign in entirely, the two minute test that decides between the carriers.

The convert and import carrier when WLM cannot sign in: WLM stores mail as EML files in a directory tree under %userprofile%\AppData\Local\Microsoft\Windows Live Mail, copyable wholesale. An EML to PST converter, SysTools EML Converter keeping the directory tree as folders, turns each stalwart's tree into a PST, which then enters 365 through attached Outlook dragging or the compliance import per the volume, the standard second stage this forum's import threads cover. This route also suits doing the migration for the stalwarts rather than with them, the trees copied off their machines and returned as finished mailboxes.

Contacts close it out either way: WLM's contacts view exports CSV, OWA's People imports it, five minutes per stalwart and WLM retires with the machine knowing its mail outlived it.