You should be able to assign object IDs to objects in your custom map and then tell the AI to create a path to that object. There are various ways to tell it to cast rays to "see" and navigate objects in its path with the ability to place waypoints along the top surface of terrain.
It's a lot of hand-holding and who knows if the server will crash with all that workload.
The battlefieldbehavior AI that uses default map terrain performs so much better. I don't know how it works and why it doesn't work for custom terrain.