Dear Wolfgang, 

thanks for your mail, that led me to the source in syst-aux where I found the TeX approach, my final solution.

\def\startCitation%
  { \startnarrower[left,right]
    \symbol[leftquotation]
    \removeunwantedspaces}

\def\stopCitation#1\par%
  { \removeunwantedspaces
    \symbol[rightquotation]
    #1
    \stopnarrower}
  
\starttext
  
\startCitation
  \input knuth
\stopCitation
\footnote{This is a Quote of Donald Knuth}

\input knuth

\stoptext

On 26 Oct 2016, at 18:40, Wolfgang Schuster <schuster.wolfgang@gmail.com> wrote:

26. Oktober 2016 um 16:30
after some readings in the TeX book I came up with the following solution that works for me without having to change all my instances of startstopCitation in the text.
This doesn’t work when you have more than one paragraph in the Citation environment.
\definestartstop
[Citation]
[
before={\hangindent=20mm\hangafter=0\symbol[leftquotation]},
after={\removeunwantedspaces\symbol[rightquotation]},
]

\starttext
\startCitation
\input tufte
\stopCitation
\footnote{This is a Quote of Donald Knuth}

\input tufte
\stoptext
Here is a alternative version but you need a empty line or \par after \stopCitation.

\define\startCitation
  {\startnarrower[left]
   \symbol[leftquotation]\ignorespaces}

\define\stopCitation
  {\BeforePar{\removeunwantedspaces\symbol[rightquotation]}%
   \AfterPar {\endgraf\stopnarrower}%
   \GetPar}

\starttext

\startCitation
  \input knuth
\stopCitation
\footnote{This is a Quote of Donald Knuth}

\input knuth

\stoptext

Wolfgang
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