Here is what I have so far:

FUNCTION get_ids_by_acct(acct_p NUMBER) RETURN SYS.ODCINUMBERLIST
IS

CURSOR cli_cur
IS
  SELECT id
    FROM test_clients
   WHERE account = acct_p;
     
ids_n SYS.ODCINUMBERLIST := SYS.ODCINUMBERLIST();
cnt_n NUMBER := 0;

BEGIN

FOR cli IN cli_cur LOOP
  ids_n.EXTEND;
  ids_n(ids_n.COUNT) := cli.id;
  cnt_n := cnt_n + 1;
END LOOP;

IF cnt_n = 0 THEN
  ids_n.EXTEND;
  ids_n(conts_n.COUNT) := -1;
END IF;

RETURN ids_n;

END;

I have the following function that works just fine when I call it like this:

   SELECT *
     FROM test_clients 
    WHERE id IN (SELECT * FROM TABLE(get_ids_by_acct(1)));

but I would like the same result by doing the following:

   SELECT *
     FROM test_clients 
    WHERE id IN (get_ids_by_acct(1));

is this possible?