[3dsmax] is it possible to write custom data to tangent? (.fbx)

Hello,
in Unity Mesh class, we can write and read tangent vector list using Mesh.tangents.
but I can't find a way to write custom data into tangent in 3dsmax.
I can edit normals using "edit normal" modifier, but i am looking for something similar to edit tangents also.

What I want to achieve is saving unified normal vectors to "tangent"(for edge pass in shader).
while normal storing the usual split vertex normal list with smoothing group(for rim lighting in shader).
so that I can extract the data inside Unity for my cg shader to render an edge line for my character model.

Is it possible to write custom data to tangent in 3dsmax? (which can still be saved after export to .fbx)
Do I need to know maxscript/3dsmax SDK in order to do this?
thanks!