Is it possible to add an SSH key to a droplet without using the existing keys to log in?

I think this is possible by booting the recovery kernel, mounting the filesystem, and modifying /root/.ssh/authorized_keys, but that would involve manually typing in the SSH key, as the console access doesn’t appear to support copy-pasting.

Do I understand correctly that access to my DO account can be used to gain root access to an instance how I described above, and is there any simpler way of doing so?