How do you generate NavMesh for exteriors? I know how to do it for interiors, but not for exteriors.

While you're waiting for a more direct answer, you can check out several threads on vanilla navmeshing that I placed in one topic(its kind of old)

http://www.gamesas.com/topic/1464647-the-navmesh-compendium/?hl=%2Bnavmesh+%2Bcompendium

Is it even a good idea to generate a navmesh for exteriors? My gut feeling tells me there would be so many little manual cleanups needed that you'd be better off just doing the whole thing manually.