I'd like an "elegant" way to accomplish this as well. I've added the appropriate stuff to /home/httpd/vhosts/<domain>/conf/httpd.include but of course psa clobbers this anytime I make modifications via the control panel.
The other option I haven't messed with but should actually work better (although still not "elegant") is to add a row to the subdomains table. At least then it would re-add the stanza to the httpd.include file
Next, in the file /home/httpd/vhosts/domain.com/subdomains/subdomain/conf/vhost.conf put 'ServerAlias www.subdomain.domain.com' and then run /usr/local/psa/admin/bin/websrvmng -v -a