"There is a way to put some line in the product to disable the smoothing even in the case when the smoothing option in firefly is checked on?"

Should be possible... Just had a look at a prop created and saved in Poser 5...

At the end of the prop you'll find...

}
locked 0
backfaceCull 0
visibleInReflections 1
visibleInRender 1
displacementBounds 0
shadingRate 0
smoothPolys 0
}
doc
{
addActor BackDrop
}
}

It should not be a problem adding those lines to a P4 prop.
P4 will just ignore them, P5/P6 will interpret them.

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