Dear Wolfgang,

Thank you for the solutions.

But for the question 2, I didn't explain well what I want to.

Actually, the codes

\def\circlednum#1{\uchar{36}{95+#1}}

\defineconversion[cnum][\circlednum]

working well by saying that \startitemize[cnum][stopper=] ...

And, your suggested method works well, too.

My question is that is it able to define new itemize  header, for example, circled number using \defineitemgroup so that I got  circled number in the 3rd level automatically without saying that \startitemize[cnum].

In the wiki, 

  \defineitemgroup[ltxitm][levels=5]
  %
  \setupitemgroup[ltxitm][1][1]
  \setupitemgroup[ltxitm][2][2]
  \setupitemgroup[ltxitm][3][3,packed]
  \setupitemgroup[ltxitm][4][4,packed]
  \setupitemgroup[ltxitm][5][5,packed]
  %

I tried to define the third level with "cnum", but it is not working. Some official documents has their own numbering system. In that case, it is very convenient to define new itemgroup following its rule and forget defining the symbol for each level every time.

There is no problem to use predefined symbols but I couldn't succeed to insert my defined symbol for some level.

Is there a way to do that?