Нашел решение, если кому пригодится
http://muellerstefan.de/?p=219
#select your file node that is connected to your bump2d node and run the script
from pymel.core import *
def normalStrength():
myNormal = ls (sl=True)
myBump=listConnections( myNormal[0].outAlpha, d=True )
myBump[0].bumpDepth >> myNormal[0].colorGainR
myBump[0].bumpDepth >> myNormal[0].colorGainG
myBump[0].bumpDepth >> myNormal[0].colorGainB
expression(ae=False, n=myNormal[0] + '_normalMapExpression' ,o=myNormal[0], s='colorOffsetR = (1 - colorGainR) * 0.5; \ncolorOffsetG = (1 - colorGainG) * 0.5; \ncolorOffsetB = (1 - colorGainB) * 1.0;' )
normalStrength()