How do I add a database from a Microsoft OneDrive Library

Because the native OneDrive SDK doesn't supports libraries, the best way to do this is via the Files method. You need to install the OneDrive app and to be able to view your library through the iOS Files app.

Once you can do this you can add to Strongbox. Either tap + > Add Existing > Files > Browse to your database. Otherwise you can open the file from within Files with Strongbox.