This is copied directly from autocad 2000 help, it looks like you will need to modify an .shx file
Regards Adrian
The AutoCAD SHX fonts have limited superscript and subscript capabilities. However, it is relatively easy to modify shape definition files to improve superscript and subscript capability.
Creating superscripts and subscripts requires two steps. First, the "imaginary pen" that is creating the text, vector by vector, on your screen needs to be shifted up or down. Then, the font "scale" needs to be reduced. In addition, the reverse process has to take place to return to the normal font. The font needs to recognize four new keys: two for superscripts and two for subscripts. To avoid altering the existing font definitions, you can access these with the numeric keypad on your keyboard.
The following example procedure is based on the AutoCAD Romans font file, although a similar method applies to any AutoCAD font. This procedure adds four new shape definitions to a font: super_on, super_off, sub_on, and sub_off, which control the position and size of the characters that follow. For simplicity, this example replaces the left- and right-bracket characters ([ and ]) and the left and right curly brace characters ({ and }) with the new characters. You may choose to replace other characters or use a shape number in the extended range (ASCII codes 128 through 256). If you use an extended shape number, you need to use the %%nnn method (where nnn is the ASCII value of the character) for placing the new characters
