I've concatenated values of multiple non-Marketo form fields, then passed the resulting JSON object as a value to an SFDC field through a REST API Create/Update lead call. What's your use case?
A bit of JavaScript would do this, depending on all the fields you want, what types they are, etc. "oninput" would be a trigger if you're adding values to as-you-type. If you have a native Marketo LP and Forms 2.0, you can put a rich text "field" in the form that appear to be Marketo fields, and then write JS from that to a hidden field in whatever pattern you want.
Similar to this approach
here, but use that as a building block to write JS that makes sense for your particular business case.
Best,
Edward Unthank | Founder, Etumos