Hey:
I'm pretty sure the only way to do it through the UI is to do it via forwarding (or frame forwarding). However, Plesk does have some ways for you to tweak Apache directly (if you are the server admin).
Disclaimer:
The correct way to do it is with the forwarding functionality within PSA. Following the steps below could cause unexpected behavior if the domain you want to alias to an existing Plesk domain is later added via Plesk. (Basically, whichever one shows up first as Apache parses the config file will be the virtual host container that returns the content).
With all of that being said, you can override Apache directives for a virtual host, if you have root access to the box, by creating the file vhost.conf (or vhost_ssl.conf if you want to override the SSL virtual host container for the domain). This file goes in /home/httpd/vhosts/<domain>/conf.
For example, if I have a domain data393demo.com, and I want .biz and .it to pull the site up too, I could create vhost.conf and place it in /home/httpd/vhosts/data393demo.com/conf. This file would contain the following:
ServerAlias
www.data393demo.com www.data393demo.biz www.data393demo.it
I would save the file and run the following on the command line:
[root@psa75 conf]# /usr/local/psa/admin/bin/websrvmng -v -a
[root@psa75 conf]#
The above command will place the following into the httpd.include file if the vhost.conf file exists for that domain:
...
ErrorDocument 403 /error_docs/forbidden.html
ErrorDocument 404 /error_docs/not_found.html
ErrorDocument 500 /error_docs/internal_server_error.html
Include /home/httpd/vhosts/data393demo.com/conf/vhost.conf
</VirtualHost>
Therefore, you can override any directive above the include line by placing properly formatted directives in the vhost.conf file. Be sure to test everything after you make this kind of change, and be sure that you do not mistype any directives as Apache will not restart.
Once again, I would recommend that you use forwarding, but if you really, really want to do it another way, this is one way you could go about doing it.
Good hunting...
Steve