I don't see how rsync would help with subdomains in any case.
You need to do two things to get this working. First off, you need to add a subdomain record to the DNS, which is done by the ddns service. You can either create specific subdomains, like:
sub1.domain.com. IN CNAME domain.com.
sub2.domain.com. IN CNAME domain.com.
or get everything done in one fell swoop with
*.domain.com. IN CNAME domain.com.
The second method would require your webserver to sort out the requests, and if somebody requests a subdomain that doesn't exist, your server would either need to have a default sub selected, or return an error. I would use the first method, where each subdomain has its own record.
Then on your webserver, you need to create sites for each of the subs. If they are "true" sundomains with different data, or just redirects back to domain.com is unimportant, just have your server set to read the request, ie sub1.domain.com, and sub2.domain.com, and serve away.