\unprotect

%\let\strc_itemgroups_start_no_item\strc_itemgroups_start_item_nop

\def\strc_itemgroups_start_no_item
  {\let\currentitemreference\empty
   \strc_itemgroups_increment_item_counter
  %\advance\c_strc_itemgroups_n_of_items\plusone
   \setbox\b_strc_itemgroups\emptyhbox
   \strc_itemgroups_check_for_repeated
   \ignorespaces}

\protect

\showframe[text][text]

\starttext

%\startitemize[n,repeat]
%  \item Item 1 \startitemize[a] \item Item 1.a. \item Item 1.b. \stopitemize
%  \item Item 2 \startitemize[a] \item Item 2.a. \item Item 2.b. \stopitemize
%\stopitemize

\startitemize[n,repeat][width=0pt]
  \noitem \startitemize[a][width=2em] \item Item 1.a. \item Item 1.b. \stopitemize
  \noitem \startitemize[a][width=2em] \item Item 2.a. \item Item 2.b. \stopitemize
\stopitemize

\stoptext