Like someone else said, your either going to have to make the appropriate texture files for use in Poser, or go the shader route to convert them to what Poser understands. Which way you go depends on which render engine you are going to use. You could go the shader only route, but that can be very complex to get to match a game engine in Firefly. Cycles nodes do know how to extract the trans layer from the TGA with a cycles image node. Cycles can also do multiple types of gradients at the same time, (world and tangent) 

In Poser I would start by using the yellow area of the face texture as just the transparency for the hair, and go from there. Based on your description you used it for diffuse in Poser, and the game engine probably just used that as the transparency, and tangent on part of the hair. With either a hair shader or a color chip for the color of the hair.

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