How can I stretch the height of the x cell (in my example) to fit the x
cell content? I have tried with extreme tables and natural tables but
without success.

\starttext
With extreme tables (y cell content fitted by default):\crlf
\startxtable
\startxrow
    \startxcell[nr=2]\input knuth   \stopxcell
    \startxcell x                          \stopxcell
\stopxrow
\startxrow
    \startxcell y                          \stopxcell
\stopxrow
\stopxtable
With natural tables (no fit):\crlf
\startTABLE
\NC[nr=2]\input knuth   \NC x   \NC \NR
                                  \NC y   \NC \NR
\stopTABLE
\stoptext