For convenience mostly, I sometimes group together related custom columns into user categories. Note that 'Fanfic Status' column and 'Status' category is identical items here:
However, the only way to have Calibre automatically add new items is the (somewhat hidden) option in Grouped Searches. This isn't the intended use and I don't think you can add it as a hierarchical sub-category.
So my suggestion, if technically feasible: The option to set 'All Items' into the user category, and let Calibre manage it.
Note that while 'Fanfic Status' is only a few items, the making of this post was triggered by me adding over 100+ items (all from the same column) to a different category.
This situation is a good part of the reasoning behind two of my enhancement requests:
hiding items already added to the category (implemented) and
being able to shift+click to select multiple items (not implemented).
Is this something other users would find useful?