Can weapons be poisoned after crafting/finding them? The weapon I lost in this question was a very effective poisoned spear, and I'm wondering what happened or if I can get a spear and re-poison it.
At the moment, no. Weapons' stats are locked in on creation. The only way to change them is to directly edit your save file.
But, the game is currently in the very early stages, so such a feature might eventually be added.