Depends on why you're creating the subdomain. What are you using it for?
You can simply create a subdomain to point to a different server by simply adding either a zone file to the same server serving DNS for the mai zone, or by adding recors to the zone file for the main domain if hosed elsewhere.
If you need mail only, but not anything else, then create the subdomain as a domain and then either delete the public_html and private_html folders, or leave them alone.
Jeff