Alter-Fox is correct - you just pick the "Static" AI option and you're done.
Because static objects are basically robots, you can have
1) As many static objects in a level as you can have robots (in D2X-XL, this is a lot - thousands I think)
2) As many unique kinds of static objects as D2 has types of robots (somewhere around 70 I think).
To make these, you basically just have to make a custom model that looks like part of the level rather than a moving robot - you can use tools like Polytron for this, but you can also make them in a 3d model program of your choice and convert them to .ASE format. D2X-XL can load these directly (the details on how this works can be found on descent2.de).
I have not confirmed that they appear in the Anarchy game mode. They might or might not (obviously I hope they do in this case
). There is a flag I saw in DLE-XP that might do this though...
The screenshots are taken from within D2X-XL (I did disable the HUD, that was all though). I actually had lightmaps turned off to reduce loading time between iterations, so it would probably look a little better on higher quality settings. The background is indeed a skybox - it isn't practical to do much of that without one.