Assuming you have already adjusted OneDrive settings for the most economic use of your hard drive (see how here), you can use the "mklink /j" command to link your folders on an external drive onto OneDrive. The "mklink /j" command makes a directory junction in Windows 10.
Assume I have folder D:\myfiles\ on my SDXC.
1. Create a folder on OneDrive which will sync with the "myfiles" folder and all its sub-folders. For example, Dfiles
2. Open a DOS command prompt ("cmd"), and ...
3. mklink /j "%UserProfile%\OneDrive\Dfiles" "D:\myfiles\"
OneDrive will begin syncing your files from D:\myfiles\ to the Onedrive folder.
to undo (remove or delete) this directory junction, you would run command: