The standard Arial Unicode font has many circled characters. For example, it has circled numbers (1 through 20) in positions 2460 through 2473. Other fonts have these same entries, too.
So, in many applications (including NX, I think), you can type 2473 and then press Alt+X, and you'll get a circled number 20. If that doesn't work, try copy/paste from the Windows Character Map app.