\version "2.18.0"

gtrharmony = \chordmode {
  \semiGermanChords
  c1:9^7
  c1:9
  c1:7
}

chExceptionMusic = {
  <c e g d' >1-\markup { \super "add9"} 
}

% Convert music to list and prepend to existing exceptions.
chExceptions = #( append
                  ( sequential-music-to-chord-exceptions chExceptionMusic #t)
                  ignatzekExceptions
                  )

\score {
  \context ChordNames = "chords" \gtrharmony
  
  \layout {
    \context {
      \Score
    }
    \context {
      \ChordNames
      chordNameExceptions = #chExceptions 
    }
  }
}