So, if I understand correctly, you need one full second (possibly a bit longer) for your animations. What I would personally do, to same the amount of sprites needed for the sprite sheet would be get your animation to where you want it, and move the frames back so that one second is 24 frames, 30, or 60 depending on the amount of frames your game will be checking per second, and then add the keyframes in to each. I know what your saying that that is a pain, but at most thats 60 images for each sprite sheet, where you didn't have to adjust each object 60 times but just a few times. That's how I see it anyhow. I'm curious as to what other way you were hoping for this program to handle your animations. (not in a condescending tone, im serious)
Good luck!