If I understand you correctly then yes you can add a subdomain as a domain.
Same if you use for example a example.dyndns.com then dyndns.com is not from you, but example.dyndns.com is en you can point dyndns.com nameservers for you subdomain to your directadmin account.
Then you register your subdomain as if it were a domain in the control panel, so not dyndns.com but fully example.dyndns.com and that should work.
I got a kind like subdomain active like this.
If you mean something else, please be more specific or give an example, so we better understand what you mean.