It seems for additive animations to work you need that there’s at least a non additive animation running and the additive animations themselves must run too (pause does not work as you noticed).
So, when you want to pause the walking animation, you should run a single frame instead. And for the additive animation, you should also run the single frame you are interested in, instead of pausing the anim and using goToFrame:
https://playground.babylonjs.com/#IQN716#304
Note: the first animation of a glb file is automatically played at load time. Here it is the head animation, so we must first stop it so that headSize.start(true, 1, headSize.to * size, headSize.to * size); does work (calling start when the animation is already started has no effect). Note also that for some reason setWeightForAllAnimatables(1); must be called again each time we stop/start an animation.