Customize "Format" menu bar in Yosemite Mail?

I'd like to customize the "Format" menu options that are included by default in Yosemite's Mail app. In particular, I'd like to be able to add "strikethrough" to the options that are listed there (bold, italics etc). The preference panel allows one to customize the rest of the menu, but I have been unable to find how to add / remove the default options for the Format menu itself.

Any ideas on how I could do this?