Contact form 7 is one of my favorite plugins , now after I’ve said that you can either create your own module to add a filed of “readonly” by following this tutorial http://blog.ekynoxe.com/2010/09/06/extending-contact-form-7/
or maybe even digging through Contact Form 7 Modules plugin to see how to add new modules,
but a much easier solution would be to create a regular text filed and use JQuery to add the readonly attribute
$('#inputId').attr('readonly', true);