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jecilop - To add an attachment, get yourself into Advanced mode and click on the paper-clip icon in the top row of icons.
AFAIK - the Job Spy mega-meta helper tool only works on the Tags column, and Tag-like custom columns.
The command line tools are invoked from the Windows command line. But if you're not familiar with using command line tools, my suggestion probably isn't worth pursuing. You would need to use other tools to transform what comes out of calibredb list_categories into something ebook-meta can process.
Does the object have to be the authors column, could it be a 'real' copy of the Authors column. A custom column with type names (e.g. Writer/#writers) would have a Manage items: Writers column, a'la:
BR