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).
Adobe® Content Server 4 software is a robust server solution that digitally protects PDF and reflowable EPUB eBooks for Adobe Digital Editions software and supported mobile devices, including the E Ink, smartphones, and tablet devices.
More info from Wikipedia.