How can I add subdomains to letsencrypt using certbots?

I have certbot installed and successfully use it to encrypt my homepage. Now I tried to set up an email system for my website using dovecot and postfix. I got it mostly running; the only problem is that thunderbird gives me a warning about the address being fraudulent because I use the SSL key of mysite.com for imap.mysite.com (same for SMTP). How can I add imap.mysite.com and smtp.mysite.com to the existing mysite.com certificate using certbot in order to avoid the warning?