addl. normal staff (ossia) in drummode or DrumStaff context

[Uwe]

Dear lilypond people, 

I have just started to use lilypond to set some drum scores. As I need to add a
few patches of backing vocals, I'd attempted to add extra (normal) staffs at
those places, however, lilypond seems to refuse doing that. 
Example: 
\score {
  \new DrumStaff <<
      \drummode {
           bd4 sn bd bd4  <<
              { bd  bd8 sn sn4 bd }
              \new Staff { c8 d8 e8 f8 g4 c4 }  >>
      }   >>
}

This produces an error like
t.ly:23:27: error: syntax error, unexpected STRING
              \new Staff { 
                           c8 d8 e8 f8 g4 c4 } 
t.ly:18:2: error: errors found, ignoring music expression

I suppose that either the \drummode or the \DrumStaff context do not allow 
this. 
Is there a way to switch these contexts off locally? 
Or is there anything else I could do to get this done ?

All good ideas or helping advices are welcome.

Uwe