Since Particle System with sub emitter can be serialized into a simple json file, why can we just edit it then export in NME? For example, we can expose “subemitter” property in NME. I believe this will make artists work better on creating particle systems. 
Unfortunately, subemiiters are controlled by code and not by shaders.
NME goal is to edit shaders only. I understand your point but the recommended way remains to edit only shaders in NME
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I understand.
In fact, I have a new artist in my team while he know nothing about coding. He wants to create particle systems by something like Unreal Blueprints.
I find graph editor in Babylon Editor, but I didn’t see any button to open it. Can you provide me some hints so I can develop an particle system editor based on it?
You can develop the shader of the particle system but the particle system itself will have to be done either in code or using the Inspector
The Particle Editor | Babylon.js Documentation (babylonjs.com)