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screen-shot-2018-02-12-at-171152.png@Adrian Ming
I am new to this process but I would do the following:
Export opacity greyscale from SP.
In Maya uncheck opaque from the shape node.
For an aiStandardSurface shader then plug the map into Transmission weight and make sure it is set to Raw and Alpha is Luminance is checked.
It works for me but there may be a better way from a more experienced person. If you find the map is inverted then swap it around under effects.