Now if only I knew how to mod in Kobold caravans. *Strokes beard*
Remove [ITEM_THIEF] or something.
I was reading up on this earlier, while searching out how to make them survive world-gen. It's actually more difficult than that, since the game still sees them as, apparently "enemies by default" if all you do is remove that tag. It's the same problem with ensuring you're at peace with goblins and therefore able to trade with them. It can be done, but I've not bothered to try since I don't know when that caravan would arrive (either season wise, or years-after-embark wise), nor do I see kobolds as being the kind of society that engages in commerce. They're scavengers that live off what they can steal or pry from their neighbors/victims; the concept of trade would probably be foreign to them.
Even if it wasn't, all they have to trade is the stuff they've stolen from other people, hardly a reliable source of regular trade goods. I could see them opening markets and being involved in black market trade (again, assuming they can even grasp the concept of commerce and trade to begin with), but I doubt kobold societies as a whole would PRODUCE anything of value in the amounts needed to run a trade caravan. Perhaps a lone enlightened kobold might find there's more money in trading than thieving and eventually become a merchant prince, but I see that as an extreme exception.