Sure, it's still possible...
You can use umm... I forgot the code but I think it should be:
ColorOps[X]=Add;//Where X is the stage number range from 0 to number of layers
You can change Add with Multiply, Multiply2X or Multiply4X...
Hope it helps
[EDIT]
You can also add a bump map using
ColorOps[X]=DOT3//Or something like that
Sorry it's been a while since I do 3DGS and shaders...