In unity, making a navmesh for a tiny planet?

In unity, making a navmesh for a tiny planet?

I'm trying to build a scene in a small planet, and baking will always give me the result on the half top of the planet. But what I want is to create a navmesh all around the small planet. 
I am wondering if there is a way to convert a mesh that I well built in blender, to be a navmesh in Unity. That means using the mesh totally without baking. Or, if there is some other ways to create navmesh for my poor planet.

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