I had a similar issue with Contact Form 7. I wanted to track each lead with a unique ID to each visitor but there was no way to add it in WordPress and thus I was out of luck. Unfortunately I am not sure if you will be able to implement the code you want to add within WordPress either (unless CF7 adds this functionality or a 3rd party creates a plugin for CF7 to do so).
That being said, we are currently working on a WordPress plugin for Convertable, which is our free lead tracking tool. It's just as easy as CF7 to build and add a form to your site, and then it automatically tracks the source of every lead (not just AdWords!) and attaches it to the lead so you can update it through the sales funnel. The leads are also stored in an online dashboard so you can follow up with the leads and then update the status of the leads. We hope the Convertable WP plugin will be added to the WP Plugin Repository as soon as next week, so I will update this thread when it is ready. In the meantime, if you feel so inclined, you can check out the site to learn more. Or keep an eye on this page which will be pushed live when the plugin is launched (currently the Download button does not work) http://convertable.com/wordpress/