Can materials be assigned to specific prims with a mesh?

Exporting multiple objects as one mesh using Houdini Engine. Need to differentiate each object (6 poly boxes), so need a per-poly way to assign material instances. Is this possible, and if so, how? I am trying to use the ‘unreal_material’ attribute.

Bonus question: are ‘prim’ and ‘polygon’ the same thing in UE?