I don't know what files are used and really how to make meshes in SR3 but I have 3d modelling programs...
But I doubt they work for SR3 anyways...
<<Edit>>
Been browsing the files...
With the tools here it's impossible...
You can't edit a mesh, unless you can save it as a ".gcmesh_pc" file (if I'm right lol...), which is impossible for the moment...
Such a pity, that we can't edit meshes/models with just Zmodeler or Blender...