I decided to make it a food recipe instead. It looks a bit strange to cook up the poison, but I prefer that over hammering it together at the forge.
If anyone knows a way to combine certain objects into a certain poison at the alchemy table I'd love to hear about it.
The alchemy table uses ingedients (in the ingredients category of the CK). Each ingredient has 4 effects. If two ingedients have the same effect then you can make a potion or poison out of them.
If you use custom items for your poison it would be pretty easy to create the poison at the alchemy table. Simply make the custom items ingredients and give them the same effect for all 4 effects (your poison effect). If you want your poison to consist of more than two ingredients then that's not possible.