Can iCloud drive be used for sharing documents produced by third-party apps?
To take advantages of iCloud Drive, the third-party apps need to implement the Document Provider Extension so you can share those files and send them to iCloud Drive. I’ve explained it in one of the post I’ve written:

Not to confuse document provider extension with share extension where you can only send and upload selected files, document provider extension provides more options to export, import, open, and move files between document storage providers.
The basic functionality each document provider extension supports is export and import. It lets the extension to export or import the files to destination path. To see this extension in action, you may update to the latest Dropbox and turn it on from the More button in Files.

Basically, you can share files from Microsoft Office to iCloud Drive as long they support Share Extension Point or the Export, Import in Document Provider Extension Point.