The UK GW site (easier to find specific articles on the UK site) has pictures of the tyranid biomorphs and how they're supposed to be modelled. I don't know how they are treated with actual WYSIWYG rules but it gives a decent reference. Look in the Tyranids section.
I useed one of the additional carapace plates to show both Reinforced chitin and extended carapace (and incidentally spinebanks), I sat it on the chest of the 'Fex, or you could possibly use the smaller extra plating in select locations, and just make sure that the other guy knows that they are reinforced and not extended
I built my 'Fex with the Spinebank plate, then took the smooth carapace plate, split it in half down the middle, reshaped it a bit to fit better, and attached it to the sides of the ribcage like an extra armored plate (leaving the little spiked ridge showing down the center). Mine's built mostly for show (don't play nids), but in game terms it'll count as reinforced, extended, or both.
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