The author link is for the author, whatever it references is available for all his/her books so it only has to be entered once. You can't add columns at the author level, the columns you see in Manage Authors is all there is
The Author link can only reference one thing, so if you put a Twitter url in there you loose the wikiP page etc. An EN Note is a container into which you can put whatever you like. It took me longer to prepare this screen shot than it did to create the note, drop the links into it, and coffee and pasta its link into the Author Link fie;d in Manage Authors
BR