Dynamically add field to Views result row

i use views_data_export to export a list of nodes to excel. These nodes of type "proposition" have many paragraphs fields, with multiple values.

By default, views_data_export export all paragraphs values in only one cell per field. I want to split these multiple values in multiple columns. One value per column.

I've written a hook_views_pre_render function :

function ehess_propositions_4_views_pre_render(Drupal\views\ViewExecutable $view) {

    if ($view->id() == 'propositions' && $view->current_display == 'export_xls') {
            foreach ($view->result as $key => $value) {
                var_dump($value);
                $index = 0;
                #var_dump($value->_entity);
                if ($value->_entity->hasField('field_participants')) {
                        foreach ($value->_entity->get('field_participants') as $key => $participant) {
                                $index++;
                                #$view->result[$key]->_entity->set("participant_${index}", $participant);
                                $value->_entity->set("participant_${index}", $participant);
                        }
                }
                var_dump($value);

        }
    }
    var_dump($view);
    
}

With no luck!

Have I to declare my new fields in a hook_views_data_alter function?