i just create a Pull Request with this feature.
In order to use the extra fields functinality you need to pass one extra parameter to the set_relation_n_n function indicating that you want to edit the extra fields
$crud->set_relation_n_n('secialmedia', 'ecommerce_socialmedia', 'socialmedia', 'id', 'id_socialmedia', 'name', null, null, true);
You can also indicate that you don't want to edit one of the extra fields of the relation table by using the unset_edit_fields. You will however be required to prepend the name of the field you wish to exclude, with the string extra_field in order to avoid name conflicts
$crud->unset_edit_fields(array('extra_field_{relation_table_field_name}'));
Follow the link