If you want to add a new frame in a specific location (not at the end of the current animation) you have to manually shift over every other frame and its settings, which - depending on the animation - can take a lot of time.
Sprite Parts already have a "Move Up/Down" function, which might be able to be used to make one for the animation step index.
So i believe a "Move Up/Down" function for step index in animations would be quite useful