Ammo can have own effect (f.ex. AP ammo) but unfortunately you can't add same type effects to ammo as you can to weapon. So the weapon can cause limb damage and poisons the target and ammo modifiers can change Damage, DT, DR, Fatigue, Spread and weapon condition with add, multiply and subtract operators. So I don't see a way to make different kinds of poison ammo but there could be different quality level darts that do more damage or pierce armor easier etc.
This is a bit above my pay-grade, but what I've gathered from other discussions, wouldn't it be plausible to make ammo that has an explosive projectile with an invisible and extremely small radius, that then applies an effect?
...somehow or another I'd be extremely surprised if there wasn't a way to create ammo that applied a posion rather than the weapon - just a matter of how far out of the box one needs to think to do it.
Then creating the ammo is just a matter of a crafting recepie: 5 darts + 1 dose of poison = 5 poison darts, etc.