There is an option in SAPScript to make text subscript or superscript. To apply subscript, change the character formats of the form in SE71, press 'character format' button and activate the 'subscript' 'On' radio button.
When you define a Sapscript form, you can define 'Character formats', by which you can change the font of some characters of your text. To get the numbers of a chemical formula for example you can define a character format which uses a smaller font size than the normal text, and which uses the flag 'Subscript = On'. This flag sees to it that the text is moved down half a line. After that you should use this character format for the numbers of the formula.
Additionaly, for using a character format like 'Subscript' directly through an application like MM02, first go into the text editor and Goto -> Header. Now you can see, which is the text object of the text, e.g. for basis data text in MM02:
Text Object MATERIAL
Then go to transaction SE75, Text Objects and IDs, and go to the line of the text object. Here you can select, which Sapscript form and style is used by the editor. If no form is selected here, SYSTEM is used as the default form. If you change this setting, it effects all new texts (so you should copy SYSTEM to ZSYSTEM for example and use this).