Hi Bab,
The migration method is so easy and it should be done per user basis.
However, as a Global admin, you are able to migrate files for your end users as long as you have full control permission on user's site. For example, I want to migrate User a2's local files to his OneDrive for Business library. Then I go to Office 365 admin center -> Active user -> tick a2 ->OneDrive settings -> click access files.
After I log into user a2's OneDrive for Business library, i copy his OneDrive for Business site URL, such as https://domain-my.sharepoint.com/personal/a2\_domain\_onmicrosoft\_com -> launch SharePoint Management tool -> type into your credential (Global admin) -> follow the prompt and type user a2's personal site URL above. Then you can migrate files.
Regards,
Sky