\setupitemgroup[itemize][1][n]
\setupitemgroup[itemize][2][a]

\starttext

\startitemize
    \startitem One
        \startitemize
            \item First
            \item[second] Second
        \stopitemize
    \stopitem
    \startitem Two \stopitem
\stopitemize

In \in[second] we \unknown

\stoptext
