RSS feed and namespaces?

Hi,

Would really appreciate if someone could tell me how I can add a namespace to my RSS feed.
I
- am using a ditto call with a tpl for each item
- have set the template to blank
- have set the media type to text/html or application/rss+xml

However, in order to stick to the RSS2 standards, I have introduced a namespace for some custom items but need to register this namespace in the header for the RSS feed but can't see how I can do this. Any ideas?

Thanks