Is it possible, if so how, to add additional categories (columns) to the playlist window?  I know you can right-click on the current headers and select from a short list, but is it possible to add to that list?  Also can you map the new columns to custom tags in the audio file or do they only exist in the local database?

Thanks in advance

No, you cannot without changing the source code.

That would sure be a nice feature to add.  That is the ability to extend the schema and then map each element in the schema back to the local database or a TAG..