Why do you need to add an SSH key? Keys are only used for authentication when doing git push on balenaCloud, which openBalena doesn’t support as a deployment method anyway.
That said, we recently added SSH keys into the openBalena API because they’re going to be used for SSHing into devices, but we haven’t published an openBalena version with these changes yet. (Nor should you manually update the API component, as it will most likely break your DB as it stands right now.)
Regarding env and config vars, the forum thread Lorenzo linked above should help, particularly this one that shows a workaround: Support for device environment variables