the only way I know to do that is not that great. It MAY work though. Basically, a blend shape has a 0-1 float slider. You could build a shader network that has all of those normal maps going into a layers or blend node and attach the blendshape sliders to the amount of texture blend. You'd have to use a bunch of multiply/divides to do it perfectly... I don't have time right now to set it up but here are the pertinent threads. If you want a simple solution that is limited in precision, here's this:

https://www.3dbuzz.com/vbforum/showth...l-maps-in-Maya

The first suggestions seem to work for many folks.

Of course, the better way is to include muliply/divide nodes to do some crazy transformations of your normal maps so they can all be combined properly AND kept underneath SuperRGB scales (underneath 1.0/255,256 depending on your color mode). Some of the calculations necessary are contained here:

https://www.rodgreen.com/?p=4

It would take quite an interesting shading network to get multiple normal maps to combine perfectly. If I weren't in the middle of a big project I'd try to build one. But hopefully this'll start you on the right path if you wanna try for the holy grail