How to "copy" with overrides in a locale file?

[Petter Reinholdtsen]

When I write locales, I would like to copy most content from another
locale, and only override the parts of the LC_TIME section containing
week day and month names.  But this do not seem to be possible.  Why?

I would like something like this:

  LC_TIME
  copy "no_NO"
  abday "su ";"må ";"ty ";"on ";"to ";"fr ";"lau"
  LC_TIME

This would fetch the formatting and other parts from the no_NO locale,
and override the abday keyword with new content.