The problem is that it doesn't seem to add space, only to be accessible as a folder. We can't use Windows Storage Spaces on boot drives either.
Enter StableBit DrivePool. On my ASUS Windows tablet I have 40GB free on C: and only 2GB free on my 64GB microSD, where OneDrive is stored (family photos and videos use almost 60GB there).
Last night I changed OneDrive on this tablet to make everything available online only, and after it finished, I moved it back to the main storage on C:... I formatted the microSD again and installed StableBit. After creating the storage pool (C: and new microSD partition) it shows a D: with 109GB available. At the moment moving the OneDrive back to D: and copying content over the network so it can get back to "available in offline mode" later.
If you don't know StoragePool works in a similar way to the old Windows Home Server Drive Extender technology.
Very impressed and happy.