There are two ways to do it, the easy way is to just use the proper unicode character. You can find the squared and cubed characters in the CharMap for the font (Win+R, type charmap then hit enter, find the character, copy and then paste in your Items collection). (Or, you can hold the ALT key down then type 0178 for the superscript 2, or ALT+0179 for the superscript 3, but this only works if you have a numeric keypad).
The other way to do it involves custom drawing each listbox item and is much more difficult. The listbox has a set of routines you can override or implement that allows custom drawing of the listbox items.
See this for an example: