Hmm so I can preview domains without dns and I cant subdomains I don’t believe it ;-)
That's only if the domain is on a dedicated IP, and that domain is set as the default domain of the IP.
The only way you can view a subdomain (or any domain) if it's not resolved is:
1) If the subdomain is just like a regular domain (setup as sub.domain.com). The account that the domain is on would have to have a dedicated IP address available, and you'd have to set the dedicated IP address as a default for your subdomain.
2) You use Plesk's 'Site Preview' feature (only available when you're logged in).