It's called sideloading, and it works for drm free ePUBS. I have my Calibre library as a Sugar Sync folder. In the Kobo app, I click on import books, and then choose the book I want. Other folks do it over USB as a cut and past/drag and drop. Folks use DropBox, too.
For books with Adobe ADE, use another reader program that supports it, like Aldiko.