The plugin will only merge Formidable field data, not WordPress user data. User meta data would need to be saved to a Formidable text field.
The “Limit Field Map to use Dropdown box” setting, limits the ‘selectable’ layouts in the “Field Map to use:” dropdown, only to the layouts that are associated with the form that is selected in the “Select the form to export data from:” dropdown. In other words, it just prevents you from selecting a form and a layout that do not work together.