I’m not sure that is possible here. You would need to have wildcard DNS entries for subdomains, and mod_rewrite rules. I found some examples when I googled, but they all seemed to say that some form of DNS editing was necessary. You could fire an email off to support and ask them.
There is some info in the old Knowledge Base:
Try to ask support…
Even if support would add the DNS record for you, it still wouldn’t work - you’d just get a bad_httpd_conf error message. The only circumstance it may work in is if you have a unique IP address.