please have a look at the IS Built-In-Services Reference and/or the WebService Development Guide for further informations.
When you have an XSD or WSDL for your Service Definition you can import this via Designer to create a DocumentType for this which has the namespace definition included.
Some services in IS public package have an input field where you can place namespace declarations as key/value pairs.
The prefix will be the key and the namespace itself will be the value then.