When you say "Office365 file", I assume you mean an OST file.
Are you trying to convert email messages? Are they from a webmail service such as gmail? You don't need to convert them. You just need to connect your webmail account to Outlook, in Office 365, and let Outlook do all the work for you.
If they are not from a webmail service, you can keep using your same old PST file, also with Office 365.