Any alternative to Navmesh's Off-mesh links for Unity Free?

Is there some code/asset I can use to make system like off-mesh links that would work in harmony with NavMesh? Or am I on my own to try to develop such code?

I think you stand on your own, but you could possibly use RAIN{AI} from Rival{Theory}:

It supports both, unity free and pro and has it’s own navigation meshes, links and you also don’t have to write the whole AI by yourself.