Awesome, I managed to get this answered by Tony on stream.
To summarize:
You can’t define paths per-say and they can’t reasonably be modded in.
At the moment the implemented path finding is A* which finds the shortest path from A to B, they plan to change to to a hierarchial model and different types of terrain have different weights, so a rocky mountain will not be chosen over a grassy route just because it’s technically shorter.
You will be able to mod these weights, so you could technically create an impassible type of terrain. [citation needed]
Edit
Quick thought, if you can dynamically set the path weight of voxel then a mod could essentially create “funnels”.
For example you could check which side of the voxel the civ is and give it a low weight where as if they were on the other side it would be a high weight, effecitvely being a one way system.
Or you could say, define a building zone where unless the civ is carrying out a building action the weight would prevent them from going through the building zone (unless there was no other way to get to where they want to go)
Man, I want to make this mod now 