how to add a domain to an existing SSL certificate on aws

AWS ELB/ALB support up to 25 certificates now. You can request new ACM certificates or upload your own certificates and use them with your load balancers

Actually (as of Feb 12, 2019) you can request another certificate and then Add to your load balancer. I just did this myself. I had one certificate with 5 domains and I didn't want to have to create another just for one more. So I created the new certificate for the one domain and then added ( look for a plus sign ). Easy and it takes effect right away. Good luck.

It is not possible to do either of these things.

Certificates can never be modified -- that would invalidate them.

Balancers cannot attach more than one certificate to a given listener, and can't have more than one listener on a port.

Your solution is to create a new certificate with all of the needed domain names, and swap them out.