Passing a variable into Contact Form 7 [closed]

Contact Form 7 has a lot of useful hooks you can use to tie custom code into it at various points of the form submission process. They are tragically not well documented at all but you can find them by googling around. You probably want wpcf7_before_send_mail. Something like this in functions.php:

function do_stuff($wpcf7) {
    // do whatever you need to the $wpcf7 data object
    return $wpcf7;
}
add_action('wpcf7_before_send_mail', 'do_stuff');