There should be a facility to direct import data from other popular third party cloud storage like google drive, Dropbox etc..
Steven Alls commented
Good news - there is! It's called MultCloud. (there's no "i" in there.)
Go to MultCloud.com and you can see the free and paid options.
I have to believe that with Microsoft screwing over people who bought into "unlimited" storage plans, there will be quite a rush to services like MultCloud for help in exporting our data to services that don't have a history of screwing over their users. (It's taken all of my bandwidth and nearly a year to upload 5TB - I was ready for a slight price increase, not losing data.)