\version "2.18.2"

#(define typewriter '((key default #f  0)
                      (cr default #f -2)
                      (bell default #f 2)))

drumPitchNames.key = #'key
drumPitchNames.cr = #'cr
drumPitchNames.bell = #'bell

% How can I append those bellow to the default list?
% (Instead of replacing the old list)
midiDrumPitches = #`(
                      (key . ,(ly:make-pitch 0 6  FLAT))
                      (cr . ,(ly:make-pitch 1 1 NATURAL))
                      (bell . ,(ly:make-pitch 1 5 NATURAL))
                      )

\score {
  \new DrumStaff
  \new DrumVoice \with {
    \consists "Balloon_engraver"
  } \drummode {

    \override Staff.StaffSymbol.line-positions = #'(0)
    \override Staff.BarLine.bar-extent = #'(-2 . 2)
    \override BalloonTextItem.annotation-balloon = ##f
    \set DrumStaff.drumPitchTable = #(alist->hash-table midiDrumPitches)
    \set DrumStaff.drumStyleTable = #(alist->hash-table typewriter)


    \balloonGrobText #'NoteHead #'(-2 . -2) \markup {"Maracas"}
    key4

    \balloonGrobText #'NoteHead #'(-2 . 2) \markup {"Open triangle"}
    bell4

    \balloonGrobText #'NoteHead #'(3 . -3) \markup {"Long guiro"}
    cr4

    % Bellow is the "test" for a normal midi output
    \balloonGrobText #'NoteHead #'(2 . 3) \markup {"Snare"}
    sn4 % Is currently not heard

  }
  \layout {}
  \midi {}
}