I'm not sure what you mean. All characters in Java are Unicode characters.
You should always specify non-ASCII characters using Unicode escapes, as follows:
'\u00b2' is a superscript two, and '\u207b' is a superscript minus.
String threeToThePowerTwo = "3\u00b2";
String threeToThePowerMinusTwo = "3\u207b\u00b2";
If these characters don't display correctly, then they don't exist in the font. If you need characters that don't exist in a font, then you need to draw them from a bitmap image - you can't add characters to the font.