NavMesh Baking Problems

Hi I want to bake a NavMesh around a little planet. As you can see the mesh bakes only up to a slope of 90Deg. But I need to bake the NavMesh all around the planet structure. Is NavMesh only good for flat and plain games or is there a way to modify it?

I wonder if there is a way to use a regular mesh (handcrafted) as a NavMesh… But as far as I can see there is no way to do so. Has anyone any ideas how I could get around this baking problem? It would be very sad if I couldn’t use this cool new feature :( I would appreciate any help! Thx in advance!