I don't know about using Poser for UV maps as I don't do that, but using UV Mapper is easy enough. Also, I use UV Mapper Pro and don't know if classic can do this or not.
1- Open UV Mapper
2- Import your Shirt.obj
Now you want to start isolating parts of the shirt for uv mapping so do a basic planar along the z axis, don't split. Then with that isolate the arms up to the arm pits.
3- Grab the torso and cylindrical map along the y axis
4-Grab the sleeves and cylinrical map along the x axis

There's really more to it than that but that should get you started in the right direction.

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