You are right!
Actually I already know the Thread but it only has the Detour (Walking) Part. You need the CAI-nmgen library and adopt the code from the SDK.
(I had the code fragment and didn’t know where I took it, now I remember, it’s the Nav-Mesh-SDK-Plugin here. And that’s what happens: The GeometryBatchFactory has a mergeGeometries)
So for everyone curious into how to build your navmesh, check out @normen’s great Plugin, you literally only need copy&paste 