For how it's done, see
http://developer.wz2100.net/wiki/ModdingTilesets
This thread contains:
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A volcanic landscape mod that replaces the arizona tileset with some sort of volcanic planet terrain. Also, far from perfect so far (: And, of course, won't work with game versions prior to 3.1.
The tileset contains some new terrain textures and re-uses some in-game terrain textures from different tilesets. It also includes re-colored decals and neutral structures to fit the place.
volcanic-1-01.wz
Licensed under GPLv2 (uses base game stuff)
History: This thread started in January 2011, but the tileset posted at this day was lost

the file was placed on some file sharing service due to not fitting into forum size attachment limit and eventually died and i didn't have a copy. Now we only have some pictures of how it used to look:
Then this message was edited, and now it contains a tileset re-made from scratch. I managed to fit it into the forum attachment limit by re-using the in-game textures, so it won't hopefully be lost again (:
wz2100-20120702_083057-Arid-T1.jpg
wz2100-20120702_085109-Sk-Startup.jpg
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