Contact Form 7 don’t allow to use Font Awesome as submit button. But is very simple way to fix this. First download Font Awesome here. Go inside downloaded package and open file font-awesome.css. Now find your font in Font Awesome directory and next this same font in font-awesome.css.
Font Awesome Directory

font-awesome.css

Using
Now go to your form. Submit button must be like this: [submit „”] when f054 is a character you want to use.
Adding CSS
Now we need to add font-family in style.css. My submit button have class .wpcf7-submit.

Enjoy!