You can easily save files directly from Microsoft Office to iCloud Drive.
First open Finder, then Finder Preferences and click on Sidebar at the top of the Preferences window and place a checkmark in front of iCloud Drive in the Favorites section (if it isn't already).
If you are saving a file in Word, for example, click iCloud Drive in the sidebar in the Save dialog and then save the document to that location like you would to any other folder on your Mac. Anything saved there will automatically be synced to iCloud.
If you have existing files already saved to other folders on your Mac, you can move them from their current folder to the iCloud Drive folder in the same way that you would move them to any other location (i.e., drag and drop, cut and paste, etc)