\def\T{%
  \startitemize[joinedup,nowhite]
    \item bar
    \item baz
  \stopitemize
}

\def\U{%
  Foo
   \T
  Bar
   \T
}

\starttext
  \U

  \startitemize
    \item Item\footnote{\U}
  \stopitemize
\stoptext