If you use H-Sphere from a H-Sphere provider, your first coverage of technical support should be your provider.
If you are a H-Sphere provider, the community forums at http://forum.psoft.net/
can be an excellent source of help and information.
Now, to answer your question, yes via the DNS editor under WWW Domains in your end user control panel.
In fact, if the subdomain exists elsewhere, you don't create the subdomain by "add domain" et al; you just create an "custom A record" in the DNS editor.