So, is this an obstacle that has no entity. simply just a cutout.
If so, we currently don't have this yet, as the navmesh you are using was the first version (get it up and running).
There are many many things we have not yet made for the navmesh, and they will be added in over time over several passes like all the other features.
i will see if i can add one for the next release, if not the one after, just got a few tasks to do for this release already.