There are several restrictions for items that are classified as event items or key items. How players obtain these items is one of those restrictions. Since atma is an item that you obtain upon completing a FATE, it uses a special system, so it would be difficult to transform it into an event item. For now, there are no changes planned. Should anything change, we’ll be sure to let you all know.
Also, I’ve passed along the desire to have atma of the same type stack in your inventory as well.