# List your certificates and copy all domains of the existing certificate
certbot certificates
# Run certbot will all domains + your new domain, sepereated by a ','
certbot --certname <certname> -d example.com,www.example.com,yournewdomain.example.com
# Now certbot will ask you, if you want to expand the cert, enter 'e'
# This also works, if you want to delete a domain from the list