You're going to want to define your sculpt further before adding on armour pieces, then you'll have a more accurate base to fit the armour to. Then I'd suggest planes and the shell modifier.
That's kind of a big question, you know? The easiest way to get an answer to a question like that is to figure it out yourself.
If you want to do it in ZBrush, I could give you some hints, I suppose. The first thing you should do is take your ZSphere armature and Dynamesh it. Do some shaping on it so that it fits pretty well the shape of a naked troll. At the point where you're satisfied with how your troll body sculpt is, you can paint a mask on the body where you want the armor piece to be, go to Subtool->Extract, and use the extracted subtool as a base mesh for your armor piece.
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Also, for next time, use to embed an image.
If you want to do it in ZBrush, I could give you some hints, I suppose. The first thing you should do is take your ZSphere armature and Dynamesh it. Do some shaping on it so that it fits pretty well the shape of a naked troll. At the point where you're satisfied with how your troll body sculpt is, you can paint a mask on the body where you want the armor piece to be, go to Subtool->Extract, and use the extracted subtool as a base mesh for your armor piece.