How to move ossa staff to "in between" or "on top"?

[Eef Weenink]

Hello all, 

For a piano-score I want to have an extra staff, so the pianist can read the 
melody of the singer. 
An ossa staff only on the places where the singer is  exactually active. 
I have made this example: (see below). 

This way the extra staff always is at the bottom. What to do to get in on top 
or in between the left + right hand staff? 

Regards, Eef

%-------
\version "2.24.2"

global = {
  \key c \major
  \time 4/4
}

right = \relative c'' {
  \global
  % Muziek volgt hier.
  
  c4 c c c 
<<
{c4 c c c c c  }

\\ 
\new Staff { d d }
>>
  
}

left = \relative c' {
  \global
  % Muziek volgt hier.
  c,4 c c c c c c c c c 
}

\score {
  \new PianoStaff \with {
    instrumentName = "Piano"
  } <<
    \new Staff = "right" \with {
      midiInstrument = "acoustic grand"
    } \right
    \new Staff = "left" \with {
      midiInstrument = "acoustic grand"
    } { \clef bass \left }
  >>
  \layout { }
  \midi {
    \tempo 4=100
  }
}

%---------