Apparently it's not, I just forgot how I went about it last time I enabled them myself. The textures are present, but there's no model for them. They could be easily modded in though by opening the light models in text editors and making copies that point at the right textures. There are also unused orange and pink textures.
This may not be possible any longer if those models have been converted to binary format (Cari has been converting as many as she can to eliminate issues with decimilization in non-US localities).
Barring all else though, if you really want green lights you can just swap the names of the textures with the white or yellow ones.