If you have multiple external IPs (you're lucky) then it should be easy to do properly. You also need multiple IPs on the internal box (which everyone can do). Create additional non conflicting IPs on your server and tell ISPConfig about it. When setting up the 2nd/3rd etc server needing SSL configure it to use a new IP. At the router you would port forward each external IP to the approriate internal IP so that it points to the correct SSL server.
You can simulate the same behaviour by using different port numbers from the outside that forward to standard port 443 on the inside but go to one of the unique internal IPs. Having unique external IPs is best if you have them.