Convert polygonal cube to solid primitive cube/box

Question

I have a radiation analysis program (NOVICE by EMPC) that only accepts 3D models made up solely of solid primitives, such as cubes or "boxes".

My strategy is to "voxelize" complex models using blender's particle system, such that I can export the model as an .obj file made up of nothing but cubes. see my previous stack exchange question for context here

However, the cubes produced by blender's particle system do not appear as solid primitives in Fastrad, (another analysis program we use to check for compatibility) but are instead made of polygons. It seems that I am getting a polygonal mesh instead of a solid. see comparison here

Furthermore, when I try testing just a single blender cube in fastrad, instead of the ones produced by the particle system, I get invalid shape errors. See first and second images from this album Although the box looks solid in this case, it seems to be made of "tesselated shapes" so it is actually probably supposed to look like the cube split into triangles from earlier.

The end goal is for my cubes to appear as a box element (see fifth image in above album) rather than a "shape".

I think a large part (if not all) of the issue is that Fastrad appears to polgonize any .step/.stp file it imports (see below). However, the only other file formats Fastrad can import are iges, gdml and sysbas. Only gdml and sysbas do not have discretization as part of the import procedure.

Below are some test files that may shed some light on the problem.

• .stp file of a proper solid cube (box) made in fastrad and translated with fastrad to the step format; this is actually invalid as well (see fourth image in above album)
• .stp file of a cube made in blender (originally exported as .obj)
• .obj of an isolated blender "voxel" from a voxelated model (it's different)
• .mtl for the above .obj
• .blend of a simple model I made in catia and voxelated with blender

Is there a way I can convert my voxelated models such that the cubes are actually solid primitives? The .stp for a solid primitive box made in Fastrad is a lot larger (two or three times the number of lines) than a blender cube, so I would like to avoid writing a translator from scratch if possible.