How do you add a new field to a webform that doesn't appear in the Marketo field dropdown?
Just to clarify, you'd like to add mailing list subscribers to a list you've created, correct?
So you would either need to create a Smart List whose criteria is "Filled Out Form > Form is MAILING LIST FORM".
...or a Static List called something like "Mailing List Subscribers" and a Smart Campaign whose Smart List is "Filled Out Form > Form is MAILING LIST FORM" and whose flow is "Add to List > Mailing List Subscribers".
I just tried that it my smart list still does not appear in the dropdown. I looked at the other smart lists that are included on the mailing form but none of them have that criteria "Filled Out Form" yet they all appear on the form.
The dropdown is a separate concept -- look in Forms Editor.