\definemeasure[CAindent][12pt]
\setupindenting[yes,\measure{CAindent}]
\setupnarrower
[ left=\measure{CAindent},
right=\measure{CAindent},
middle=\measure{CAindent}]
\setupitemize[each][margin=\measure{CAindent}]
\starttext
\input knuth
\startitemize[joinedup,packed]
\item \input tufte
\startitemize[joinedup,packed]
\item \input tufte
\stopitemize
\input tufte
\stopitemize
\stoptext