Initializing Associative Array in PL/SQL
Updated answer for 18c and beyond
In 18c Oracle has introduced qualified expressions that can also be used to initialize an associative array. Example from the documentation:
DECLARE
  TYPE t_aa IS TABLE OF BOOLEAN INDEX BY PLS_INTEGER;
  v_aa1 t_aa := t_aa(1=&gt;FALSE,
                     2=&gt;TRUE,
                     3=&gt;NULL);
BEGIN
  DBMS_OUTPUT.PUT_LINE(print_bool(v_aa1(1)));
  DBMS_OUTPUT.PUT_LINE(print_bool(v_aa1(2)));
  DBMS_OUTPUT.PUT_LINE(print_bool(v_aa1(3)));
END;

See also chapter Qualified Expressions for Associative Arrays from Easy Initializing for Records and Arrays by Steven Feuerstein
Original answer upto 12c
No - there is no a short-cut syntax to initialize an associative array. Unlike varrays and nested tables associative arrays do not have collection constructors.
Instead you should write your own initializer function. See an example from Declaring Associative Array Constants.