To add new presets to Atom open the Atom plugin, and click the top right gear icon to open the menu:

From here click the option "Open Preset Folder:"

When clicked this will open a window outside of your DAW that will locate the preset folder for Atom. On Mac this will default to Atom's Presets Folder, and the same will occur for Windows.

From here double click on the the "Preset," folder, and simply drag presets you'd like to add into their respective folders, or create a new folder entirely to add the new presets to.

When closed and re-opened Atom will now display new categories (folders) and presets you've added in!