I have finally found a not perfect solution: setting manually the height of
the bar cell shrinks the foo cell (setting height=fit for the foo cell does
not work).


\starttext

\startTABLE
    \NC foo                 \NC[nr=2] \input knuth  \NC \NR
    \NC[height=28ex] bar    \NC                         \NR
\stopTABLE

\stoptext


I have chosen 28ex for the height since linespace in ConTeXt equals to
2.8ex and I need the height of 10 lines in this example to fit the knuth
paragraph (if I could find a way to get automatically the number of lines
required to fit the knuth paragraph it would be better).