Addons, including those describing planetary systems, are loaded when Celestia starts. They can't (yet?) be loaded after Celestia starts. In other words, the planetary systems have to be defined before you can go to them.
That said, there are at least two programs available that can create planetary systems for Celestia.
One is
StarGen -- a Solar System Generator
http://www.eldacur.com/~brons/NerdCorne ... arGen.html
by Brons aka JimB. aka Jim Burrows
For more info, see the discussion at
http://www.celestiaproject.net/forum/viewtopic.php?p=9329#9329
The other is a "globular cluster generator" by Rassilon, which is available in
http://www.celestiaproject.net/~rassilon/
It generates planetary systems within a globular cluster, but you can adjust its parameters so only a few stars are created.
Be sure to get all of its pieces:
cluster.zip
gc_extras_v2.zip
gc_texv2.zip
gc_texv3.zip
For more info, see the discussion at
http://www.celestiaproject.net/forum/viewtopic.php?t=1320
p.s. Since this is a Celestia topic, I'm moving this discussion out of Purgatory into the Users Forum.