I don't know what "tool/file" are you talking about that allowed you to do that with "plateau" tool, but you can just use pathing blockers if you don't want units walk in certain area.
If your map is going to be TFT then you might use normal pathing blockers, but if the map you're making will be converted to RoC then use "Line of Sight" blockers and change the field "Art - Occlusion Height" to 0.00