Terasology\src\main\java\org\terasology\world\WorldBiomeProviderImpl.java method getBiomeAt is where existing biomes get determined and theoretically you could add another one there. However, like
Nym Traveel points out, we're working on a better "world map" generation step that'll be able to divvy out biomes better, including via mods.
Then you can look at Terasology\src\main\java\org\terasology\world\generator\core\PerlinTerrainGenerator.java for some inspiration in seeing how the current biomes matter when it comes time to place actual blocks.
Rather than modify existing code (other than improving the framework/engine code itself) it'll be better to add new generators you can select at world create time than make any substantial changes to current generators