The link you cited shows a script, not a certbot plugin.
Guess you need to create a certbot plugin for FreeDNS prior to inclusion into npm.

Alternatively, you could have a look here:
https://community.letsencrypt.org/t/dns-providers-who-easily-integrate-with-lets-encrypt-dns-validation/86438

All providers that are listed with "Certbot" under "ACME Client Support" can be integrated into npm or might even already be included.

Will Python certbot plugins be acceptable? There is one that is publicly available through pip installation that works for my custom stealth domain on FreeDNS using the examples and command lines listed in the Project README.

FreeDNS Python certbot plugin project link:
https://github.com/schleuss/certbot_dns_freedns

If not, is there anyway to import an existing Let's Encrypt cert that has already been requested and setup for certbot renewal? The block on importing Let's Encrypt certs stop people from using ones they have gotten though means other than requesting through the frontend. I did a hacky workaround of adding another test cert and then replacing the pem files with symlinks to the ones in /etc/letsencrypt/live directory. I would really like to not have to result to these kind of workarounds just because this project doesn't support a plugin for my DNS providers AND blocks manual install of the Let's Encrypt certs I get though other means.

Issue is now considered stale. If you want to keep it open, please comment 👍

@bricas I just submitted a PR that would fix the current version that doesn't work for all people that are not paid subscriber. see #3750 if you can take a look please :)