Convert one mesh of different material to mesh instance

I want to write a simple converter converting the fbx format to playcanvas format.

But,I have trouble in converting one mesh with serveal materials.

for example,a simple cube fbx file which a cube with different material in six faces.
In playcanvas,there are a mesh node and 7 meshes, while in fbx just a mesh node
So, how can I detect the mesh instances by the just one mesh node in fbx?