Command/defineseparatorset

\defineseparatorset[dash][][--]

\defineenumeration
  [test]
  [text=Test,
   headstyle=bolditalic,
   numberseparatorset=dash]

\starttext

\placelist[test]

\starttest[title={Knuth}]
\input knuth
\stoptest

\startsubtest[title={Zapf}]
\input zapf
\stopsubtest

\stoptext