Help with custom tags

Hello,
I want to use several custom tag's for my audio book collection.

from what i understand to use the id3.2 custom tags they are
Txxx or T00 or Tone or something like that.

I want to add custom fields for the following
SubGenere, Author, Nareator, Book Series, Vol. , Book Title

Any pointer's would be great,
yes I have looked through the faq and I didnt see anything about adding custom tags.
Thank you in advance.

It's TXXX
Use "View > Extended Tags...", add a new field and type your desired tag name.