This paper presents an innovative pre-processing voxelisation tool designed for the improvement of electromagnetic (EM) simulation of metal composite materials, specifically in the context of horn shaped 3D printed antennas [1,2]. Traditional EM simulators struggle to accurately model hybrid structures that combine both metal and dielectric components into a single material, as they typically analyse these materials as single lossy dielectrics or single lossy metals. As such, they do not combine their physical interactions. An initial trial-and-error approach with EM simulations and measurements of horn antenna parts, as presented in this paper, revealed that the native functionalities of a commercially available simulator, CST Microwave Studio, were possible to improved employing the proposed tool.