windows server 2008 dns: adding zone for existing external domain
Solution 1:
If you are going to set up split DNS, you need to take ownership of the records in the internal zone that you need the A record for.
In theory, you should be able to add a stub zone for domainname.com and then add a record for sub.domainname.com, but in practice I have found this to be unstable in the long run.
The better solution is just to duplicate the external zone file in a primary zone on your internal DNS servers, and then change the A records that you want to point to internal IP addresses, and add any subdomains that you require.
Solution 2:
I'm not sure what it is you did, based on what you've stated in your question, but here's what you can do:
Create a new zone on your internal DNS server for the sub-domain (the child domain).
Create an empty A record (a domain A record) for the ip address that you want the subdomain to resolve to.
You don't need to do anything with external DNS, forwarders, etc., etc. Your internal server will be authoritative for the child domain only and your current third party DNS servers will continue to be authoritative for the parent domain, from the perspective of internal DNS clients, which is what I gather you're trying to accomplish.