@dasota in DO13, to append an Author, you can try Find and Replace Metadata.
There are many forms to do that, e.g. using this in the selected files:
FRMeta FIND .* REPLACE "{author};a new author" REGEX PROPERTIES=author
Don't forget to read the main post on the command page first, and check that the file extensions you want to use are defined in `doc_exts' in the script configuration.