1) use an .SSC file to define an elliptical orbit.
Elliptical orbits naturally loop back to their starting points.
or
2) use an .SSC file and .XYZ file to define an xyz trajectory
Elliptical orbits are "easy". (yeah, right)
You might take a look at some of the references I list on my main Celestia page at
http://www.lns.cornell.edu/~seb/celestia/index.html#4.0
They're all in English. Please let me know of any you locate in other languages.
Supposedly Horizons can be persuaded to generate appropriate trajectories for spacecraft, but I've never tried to do that.
I hope this helps a little.