Unfortunately, it's not possible to add another image that isn't in the sprite's "root folder". What you can do is add a transform property to a particular image. If you pass a parameter "a_a1_at = 0_a_a1" you will only be adding the transform property to the "a1" image in the "a" group of your sprite.
I'll prepare a future update with more features for FolderSprite and this is certainly one I'll keep in mind.