As stated in the title. It's actually already possible if you select it as the key before giving it an animation, however it seems like an arbitrary limitation if it was a conscious decision. Similarily, I see no reason why it should't be possible to select literally anything as a trigger-by as opposed to just pickables (which can similarily be later animated to achieve that result already). It very arbitrarily limits the possibilities of what can be achieved in the editor, at least without using what seems like a glitch.