GREP find -> Find Format (you might need to click the "More options" button) -> Basic Character Formats -> Position -> Superscript.
in the Find What field -- this will find any text in superscript. Put this in the Replace With field:
-- where <e> and </e> are your own codes, and "$0" simply means 'that which was just found". Repeat for Subscript. It will write these codes in superscript, resp. subscript, as well but that should not be a problem if you are extracting plain text only.
By the way, <e> is not the code for "exponent".