Wont be done. Design decision long ago against this to not hurt ai walking through fires.
Since the AI can already find its way around any other obstacles, what's preventing fire objects from being treated like any other objects - the AI would simply not walk through a fire and would find a way around it? Or are the fire entities different from other types of object, let's say a crate for example (being light emitters or whatever), so that it's invisible to the AI pathfinder? I've been playing a lot of Thief Gold lately, and there the AI would not step into a fire voluntarily (though I did throw a few unconscious apebeasts into one), and that game's pathfinder routines are ancient - the object model was probably completely different, though.
As it currently is, the player can use lit torches or lamps to climb up onto, or go inside a lit fireplace with impunity - if the system relies on the mapper placing a damage entity into a fire (for example, to hide the use of that old Thief staple of a secret passage hidden in a fireplace), then it will be inconsistent across maps - in some maps fires do nothing, but in maps where they are somehow important, it would do damage and then tip off the player that there's something important enough there to make the mapper place a damage entity there (if the system indeed works like that).