code: public void addSubstitutionFont(BaseFont font) {
          if (substitutionFonts == null)
              substitutionFonts = new ArrayList();
          substitutionFonts.add(font);
      }
docs: Adds a substitution font to the list. The fonts in this list will be used if the original
  font doesn't contain the needed glyphs.
  
  @param font the font