I would like to add (not remove) DRM to an EPUB file? How would I do it? Preferably on OS X.
I tried Gooogling but (all) of them are about removing DRM from EPUB.
You'll need proprietary software made for adding DRM to ePUB's.
Check out Adobe's Content Server product line, as it implements a version of Adobe's DRM "ADEPT" (Adobe Digital Experience Protection Technology).
More info from Wikipedia.