How jump between terrain without offmesh link?

I have ground and sky terrain on a rts game, sky terrain is at the top of ground terrain, i want meshagent be able to move between these terrain anywhere.

Of course, offmesh link do not work because it have to be at the edge.

 

My suggestions would be to disable the NavMeshAgent, then sample a position on the NavMesh the agent needs to transition to. Move the NavMeshAgent yourself to the sampled position and reactive the NavMeshAgent.