The models we have in PlaneShift are in the CrystalSpace 3D engine's native format. That means it will hardly be possible to convert them back to something editable if you don't know exactly how.
Simple meshes were often created using Blender. More complex models sometimes need to be made with 3DStudio Max because sceletal animations or portals between world maps are better supported by its exporter to the CS format, but not by B2CS for Blender.
If you are interested in the development of PlaneShift, you may get recruited, see the
Developer Recruitment page. But the Atomic Blue license contains a non-disclosure agreement, because the source code of the PlaneShift game engine may be OpenSource ... but the art and database content (what makes the game the game) are copyright protected. So you will probably not get access to all the original models and textures.
The release textures in DDS format are in art/materials.zip of the game installation; the items are in art/meshes.zip, the world maps in art/world.zip ... but in a more or less game-read-only format.