Question regarding Web-To-Lead-Forms

Howdy!

Two questions: 

1) Are we able to use a Web-To-Lead-Form to modify an existing Lead, rather than just creating duplicates?  If so, how?

2) How would we specifically import a Tag from a Web-To-Lead-Form?

We already have a working WTLF that creates new leads from our website.  However, we want to create a separate form that adds a Tag to a lead that already exists (or create a new lead if one doesn't exist, as it currently does).  The specific use case for this is to track Webinar attendance via a sign-up form on our site.  

Thank you!