In the Form Designer page, I don't have a SFDC custom fields tab. Not sure why. That said, my custom SF field did show up in the Template Form Fields section.
Ideally, I would like my Web Form TextArea field to map to a Notes field attached to a SF lead. Such that multiple notes could be generated for that lead from the form. Right now, I have a single custom field for my SF leads and if the customer fills out the same form later and types in a different message in the TextArea box, it overwrites the previous value in the field instead of adding a new note or appending to it.