pinging @julien-moreau , the master behind the editor
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Hey @lingliyi
It is supported but the workflow is different than creating a multimaterial and assigning it.
To create a multi-material, you’ll can click on different submeshes of your mesh in the preview panel or use the scene graph by right-clicking the mesh and selecting the wanted submesh:
Once selected, the inspector is updated showing a new tab “Sub Mesh”. If no multi material is assigned, you can click the button to create one and give it a name:
Once created, you can select the material that already exists to be assigned on the current submesh:
I’m adding this step in the documentation as I realized this part is missing 
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Did you try following @julien-moreau 's steps?