Well, there's
this obj plugin for sketchup.
After using that to export the mesh, there are many way to go on
For example [ame="http://www.orbithangar.com/searchid.php?ID=2961"]this[/ame], which will directly convert the obj file to a msh file for Orbiter. I never used it so I don't know how the results are.
You could also open that obj file you exported from sketchup in Anim8or, Wings3D or gmax, there are msh exporter for all of them.