this simple wormkit module allows you to use custom terrains without replacing the original ones
usage: put new terrain folder in Data/Level (for example: Domestic or Dino from WWP)

note: terrain lists and files are NOT synchronized in online multiplayer games! if you want to play online with your friends, ensure that you have the exact same terrains installed.

tested on steam 3.7.2.1, probably works on cd version too.
edit: updated to newer version - you can use now up to 127 terrains total

download: wkTerrain.dll
source code: dllmain.cpp

examples:
- dinos from WWP
- domestic from WWP