subdomains of subdomains?

You don't want to do that. A sub-domain account type is just an ownership concept, and has nothing to do with the domain name. I recommend against using sub-domain accounts at all...use a sub-server if you want a virtual server to be owned by an existing account, or a regular virtual server if you want a new account. Any of them can be named anything you want (sub.sub.sub.sub.domain.com can be a virtual server, a sub-server, or a sub-domain--it doesn't matter, it's just a name).

See these two docs for more on the terminology:

http://www.virtualmin.com/documentation/id,definitions_for_virtualmin_te...

http://www.virtualmin.com/documentation/id,virtualmin_for_cpanel_users/