You can make copies of different metadata entries and then manipulate the copies.
For example:
Code:
add_to_extra_valid_entries:,myao3fandoms,myao3freeformtags
include_in_myao3fandoms:fandoms
include_in_myao3freeformtags:ao3freeformtags
When doing lots of metadata work, I suggest setting up the 'Saved Metadata Column' feature if you don't already use it.
Then you can do updates with 'Update Calibre Metadata from Saved Metadata Column' without changing your ebook files or hitting the site again until you're happy with the output.