i didn't notice any related method.
No, jPCT-AE doesn't. Desktop jPCT does. I think the reason is that it wasn't supported in OpenGL ES 1.x, so I left it out when porting the thing and never added it again. You should actually be able to simulate it in a shader.
now i have another question, how can i use multiple shaders on an Object3D. i think i have read somewhere that an object can have several vertex shaders and fragment shaders in a row.
the reason i want this is that i don't like to modify the default shaders.
No, you can't do that, I'm afraid.