Awesome, I managed to get this answered by Tony on stream.
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. 
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