Currently, sf_webform_export() calls salesforce_api_fieldmap_export_create() in order to create a basic export object, after which a good deal of post-processing is done to get the object into working order before it's shipped off to salesforce.

salesforce_api_fieldmap_export_create() implements some validation and default value assignments that are not effective for sf_webform, since they are not performed against webform submission values. sf_webform, sf_webform_fieldmap_objects() (and in turn _sf_webform_get_fieldmap_object()) should populate its field declarations with 'export' and 'import' indexes, and create sf_webform_export_default() and sf_webform_import_default() handlers to populate export/import data.

For example, see _sf_node_export_cck_default().
See also hook_fieldmap_objects() documentation in salesforce/hooks.php