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this could theoretically be done by the following dictionary
in your .confix profile dictionary. otoh, I'm quite
confident that there's some ugly bug that makes life hard.

'CONFIX': {
('\.hh$', BuildableHeaderCreator())
}

code pieces involved are:

buildable_mgr.BuildableManager.register_defaults_
buildable_h.BuildableHeaderCreator.{create_from_file,is_valid_ext}

where the latter most likely makes the .confix thing hard.
that's something inherent to all buildable types, not only
C-Headers.