I explicitly wrote "adding fields is not supported" to avoid any doubts.
Actually we are not planning to introduce such a feature: few requests from our actual market and not compatible with our actual price policy.
Date, author, annotation's name are metadata related to each Annotation whose support is included in both Professional and Premium.
The approach to the thread seems really conversational and could became really heavy to be handled and unuseful for many other users.
I shall ask you to pur specific requests if you have any.
Our licenses levels are Standard, Professional and Premium.
One example about Advanced API is included in the demo project.
Some other are
www.radaeepdf.com/support/knowledge-base?view=kb&kbartid=125
www.radaeepdf.com/support/knowledge-base?view=kb&kbartid=133
www.radaeepdf.com/support/knowledge-base?view=kb&kbartid=64
and similar and related KB articles.
If you have some specific needs it could be better if you get in touch with sales team trough the trouble ticket area.