I think you’ll have to do this in your word processor after compiling. If you use Nisus Writer Pro, it would simply mean opening the style sheet, setting the “Note Reference” to non-superscripted, and putting [ before the number and ] after it in the “Footnote” style. I can’t say for Word, which I don’t have, or Mellel, which I have but don’t really use.
By the way, by “brackets”, I would assume they mean [ ], rather than parentheses, i.e. ( ).
Hope that helps.
Mark