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You have to set the TurboSmooth modifier for all the objects/groups for every smoothing level (0, 1 and 2) and export them all together in 3 separate fbx files, just don't forget to enable the Geometry > TurboSmooth checkbox in the fbx exporter. If the destination application can't interpret the TurboSmooth modifier correctly, collapse the modifiers on every object/group before exporting.
Thank you very much you helped me a lot :)
