Run it as follows
sudo ./certbot-auto certonly --standalone -d originaldomain.com -d www.originaldomain.com -d new.originaldomain.com -d new2.originaldomain.com -d new3.originaldomain.com --dry-run
Then you will see it tell you about expanding your original cert … NOTE: use the –dry-run flag for testing if it gives you the correct output without errors then re-run the same command and remove –dry-run from the end.
Also note that ./certbot-auto is the new official command, ./letsencrypt-auto has been deprecated.
Also bare in mind as I said if you want all those sub-domains to be different web sites then you should generate individual certs for them rather than expanding your main certificate.