Extend Fields - like for real

Kirby 4 Question❓
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1

I want to define the available block-types for the layout field in a central blueprint file. For example fields/blocktypes.yml looks like this:

fieldsets:
  - heading
  - text
  - list
  - image
  - gallery
  - video
  - markdown

and is reused like so:

  articleText:
    label: Haupttext
    type: layout
    icon: text
    extends: fields/blocktypes

So far, this works fine. But now I want to extend the range of selectable blocktypes in one blueprint by the code block. How would I achieve this? If I do this, the fieldset options are simply overwritten (which in most cases is desirable but not for my usecase):

  articleText:
    label: Haupttext
    type: layout
    icon: text
    extends: fields/blocks-fieldsets
    fieldsets:
      - code