You can partially do this, but it takes some work. Create your additional domains. In server-manager you can create a user as user.one, user.two, etc. Then in /home/httpd/html/horde/imp/config/conf.php, change the server_list option under //160MailServerSettings from none to shown. What you will see are the entries that are in /home/httpd/html/horde/imp/config/servers.php.
So what you will now see when you go to webmail, is an extra option at the top that says choose a mail server. User settings are stored in mysql as userid@domain.com, i.e. user.one@domain1.com, user.two@domain2.com.
Now the downside. The current architecture of SME server does not allow it to distinguish whether a user belongs to domain1 or domain2, so user.one could choose either server option and still login. What would be lost are any user specific settings made while logged in to one of the domains. So you would need to educate your users.
So, it's doable, but maybe not practical.
John Bennett