For adding special symbols under Mac OS X like m² and m³ this is possible via show Emoji & Symbols. This option can be found under the flag in the menubar. You must activate system preferences, keyboard, show keyboard, emoji, symbol viewers in menu bar. To see the flag go to system preferences, keyboard, input sources show input menu in menu bar.
To add the symbols m² and m³ search under Emoji & Symbols to 2 or 3. Here you can add ² and ³. You add this symbols in Mail or Libre Office