[basd]

I don't see an easier solution than "download and import" unless you want to actually utilize MobileSheets sync. Wouldn't you just share your DropBox, GDrive or OneDrive Mobilesheets to their DropBox, Gdrive or OneDrive account? This is what I do. Also, I made a OneDrive solely for MobileSheets, so sharing the login would not be problematic -- but nevertheless, users do not log into my OneDrive, I share the folder with their OneDrive accounts (which are free and easy to make on all of Windows, Android and iPad.) But, recently Microsoft made shared OneDrive folders "link only" and the shared folders no longer synchronize to the local computer. As far as I can tell, MobileSheets synchronize via OneDrive still works (although this change has majorly disrupted my MobileSheets setup.)

I don't see an import option that can access a web url, unless your users were to install software that would let their system file browser access the web url, and then it would have to be a bare url (no index.html) so that the directory structure would appear. I am just proposing a theoretical idea, I haven't tried it, but I know that some of my systems can mount and view an ftp or web url as though it is a drive on my computer.