Disconnected mesh import and GPS

Hi there, I’m looking for advice on importing and registering disconnected meshes in Geant4 and using them as particle sources.

For example if I take a cube (1) and perform a Boolean difference using a rectangle, the cube becomes 2 separate watertight meshes (2).

Could I import this single mesh file (e.g. STL + CADMESH or similar) containing 2 unconnected meshes into geant4? How would I register it in the DetectorConstruction? Could I generate particles within these 2 enclosed meshes without splitting them apart?

Finally, if I isolate the rectangle mesh used in the Boolean difference (i.e. the space in-between the 2 meshes) - could I register that as a detector (3) ?

Thanks in advance.