expand an existing fullchain certificate

I want to expand an existing file instead of creating a new certificate for a subdomain. I have these domains already in the file:

certtool -i < /etc/letsencrypt/live/example.org-0002/fullchain.pem|grep DNSname
                        DNSname: forum.example.com
                        DNSname: m.example.de
                        DNSname: m.example.org
                        DNSname: example.com
                        DNSname: example.de
                        DNSname: example.org
                        DNSname: wiki.example.org
                        DNSname: www.example.com
                        DNSname: www.example.de
                        DNSname: www.example.org

(certtool is part of the package gnutls-bin in debian) I know, there is the certbot --expand option, but if I don't get the exact set of domains again, it will create a new certificate with the next suffix -0003 How can I prevent that, if I only want to add one domain to an existing cert file?