Just so we're understanding each other:
You can't upload your custom content onto the official server, so whatever you do on your end of the client can only be seen by you and those you give your modifications to.
If you want to place an object model into a PlaneShift map on your end, export the model as usual, place it with the other art files, and then edit the world map you want to have it in. Extract the "world" file from the .zip you want (like hydlaa_plaza) and then convert it to XML format using the docconv utility (one of CS tools), like this:
docconv -outds=tinyxml world
Add the mesh somewhere between the <sector name="mapnamehere"></sector> tags - you can figure out how it works pretty quickly by reading through the XML, and you don't have to convert the map back into binary format at the end.
I hope this is what you meant?