atto is about the closest their writing rules allow. It is all pairs like ka, ki, ko, ku, except for n. So all loan words have to be extended if they end illegally.
Actually it's interesting that you mention this. Technically, you can also make it atoma-ku, (like ato like in later, and ma-ku as in mark). I wonder if the dip-tone was intentional to make it sound more like foreign word.
