Hi M.EContracting,
We don’t have a straight way or feature to upload PST files into Office 365 mailboxes.
For this situation, you will need to set up the O365 user accounts using the Exchange mode in the new email profiles in Outlook 2007 clients first. And then import the PST into these accounts.
After that, O365 will sync the emails from the Outlook clients to the Cloud.
NOTE: Office 365 Exchange Online is not designed to work with Office 2007 because the mainstream support of Office 2007 has been ended on October 12, 2012. Office 365 does not offer code fixes to resolve problems you encounter when using the service with Office 2007. As a result, it might be an expected behavior that the quality of the user experience will be diminished and many new Office 365 experiences will not work in Outlook 2007.
Thanks for your understanding.
Best Regards,
Henry Huang