First off, you can\'t import your own stuff into PS, because you\'d need to import them into the server map, and you can\'t do that.

All you can do is change the models and looks of current things.
How you do that, well... On the LWW site there is a MAX -> CS stuff exporter, but I have no idea if there is a CS -> MAX thing... You\'d have to ask one of the devs about guidance on such a thing.

Edit: Actually, I have no idea if those model files are even saved in CS format, but I know some of the stuff can be found throughout the PlaneShift folder as .spr files.