Right, so there are 2 options here.
The first, ask Bask mentioned, is a simple redirect. You can do this in the domain names control panel, you should be able to tell the subdomain to redirect to a website (put DDNS link here). This is best for personal use and if you're not too fussed about the DDNS name being seen.
The other is to simply make a page in the subdomain folder that redirects to the other page. Call it index.php and simply have this 1 line of code:
<? header('location:http://ddnsurl.whatever.com:port'); ?>
So in effect it's the website at the subdomain just forwarding you, rather than on a DNS level. There are other ways with DNS settings but to be honest this is the easiest solution.
Edit: Ignore the above stuff, didn't read all your posts. The above information is helpful for others anyway, but for your specific issue, it's a bit different.
What you will need is a CNAME record - this points a subdomain to a different website. In your control panel for your domain name you need to edit the DNS settings to add a CNAME record.
You will have 3 options. The "Name" is the name of the subdomain. So if you did nas.domain.com then you enter "nas" here. The Host Name is where to send it to, so the DDNS address. Leave the TTL as default.
This should work.