The 'Box' and 'Plane' primitives really are primitives. And they can build a 3D tesselated representation just for display.
If you want to build a box from some given vertices, then you should create a standard triangular mesh (ccMesh). There are a few examples of creation of custom ccMesh instances in the code. Basically you create a point cloud (with 8 vertices), then you create a mesh with this point cloud as 'vertices' and you add the triangles (as 3 indexes referring to the vertices). You can also add per-triangle normals, etc. (in this case you can take a look at how a ccBox instance creates its own representation).
Daniel, CloudCompare admin