The normalmapping sample on this site sounds pretty close to what you need: have you looked at that?
It uses a string attribute from the original FBX file to decide which texture to use as the normalmap, but that could easily be changed to pull that data from somewhere else (for instance you might just use a convention based on the filename of the diffuse map, where seeing bricks.png would cause your processor to also load bricks_normalmap.png or something like that).
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