We can't support animation controller on additive layer. There is too many edge case that won't works like root motion so we can't build a generic solution that will works for everybody.
By definition an additive layer doesn't blend the animation like an override layer and need additive animation to works.
The only supported way to generate additive animation is with an animation clip setup with a reference pose.

That been said if the user want to experiment stuff on his own he can use animation jobs with AnimationScriptPlayable and build his own additive controller

The playable connection would look like this
LayerMixer.Layer1.input <--- MyScriptPlayable <-- Controller

in your animation job for MyScriptPlayable you will need to iterate over all the animation stream values to subtract a reference pose to yield an additive animation stream.
But there is no guarantee that it will work for your use case.