Assinging materials in Creo via Weblink

Yes

Usually, You assign a material using

oSession = pfcCreate("MpfcCOMGlobal").GetProESession();  
CurModel = oSession.CurrentModel;  
CurModel.CurrentMaterial = CurModel.RetrieveMaterial(material_name);

But there is a catch.

This code line only imports the number of a certain value. The unit described in the imported *.mtl file is ignored. You have to set units manually by using:

CurModel.CurrentMaterial = CurModel.RetrieveMaterial(material_name); 
CurModel.CurrentMaterial.SetPropertyUnits(pfcCreate("pfcMaterialPropertyType").MTL_PROP_MASS_DENSITY, "kg/mm^3", false);   
CurModel.CurrentMaterial.SetPropertyUnits(pfcCreate("pfcMaterialPropertyType").MTL_PROP_YOUNG_MODULUS, "MPa", false);  
CurModel.CurrentMaterial.SetPropertyUnits(pfcCreate("pfcMaterialPropertyType").MTL_PROP_THERMAL_EXPANSION_COEFFICIENT, "/ K", false);  
CurModel.CurrentMaterial.SetPropertyUnits(pfcCreate("pfcMaterialPropertyType").MTL_PROP_SPECIFIC_HEAT, "joule / (kg K)", false);  
CurModel.CurrentMaterial.SetPropertyUnits(pfcCreate("pfcMaterialPropertyType").MTL_PROP_THERMAL_CONDUCTIVITY , "W/(m K)", false);