It seems to be very hard to add information to IfcMaterial.
In IFC4 it’s the very same concept Property Sets for Objects applies to IfcMaterial, the confusing thing here is that IfcMaterial is not an object. But it’s not that different - IfcMaterialProperties holds link to IfcMaterialDefinition with inverse HasProperties attribute.
Relevant discussion from not too long ago. With software examples.
