Using rancher-cli "Failed to find SSH key for hostname"

Rancher-cli’s ssh command is extremely handy but how can I add the SSH keys for already registered hosts? I couldn’t find anything in the documentation about that?

According to the video where the rancher-cli was introduced (on youtube), it will only work for hosts that are provisioned from the rancher server using docker-machine.

I would very much like a way to set a key for existing/externally added hosts as well.

BTW, rancher-cli is awesome. Nice job!