Please move the thread to Products as it's specific to the Marketo product platform.
After you relocate it, also clarify what you plan to do with these "additional field values."
UTM values in the current query string are already transported with any Munchkin Visit Web Page activity. The parameters of a VWP call don't automatically update lead fields, but they are certainly sent to Marketo, where they can be actioned using Smart Campaigns.
While there's a trick to update a single custom lead field to an arbitrary value, the principal means of updating multiple lead fields is the Munchkin API associateLead method, which requires a key generated securely server-side (of course the updates are done client-side). Read this blog post for more information.
A lead doesn't have a single set of UTM values over its lifetime, of course. Storing an array of all UTM "encounters" is a better model, or at least a first touch set, conversion touch set, and ongoing touch set.