Solution:

Changes in the file forms/download.xml:
Add after field event_title:
<field
            name="event_title"
            type="radio"
            label="COM_ICAGENDA_EXPORT_EVENT_TITLE_LABEL"
            default="1"
            class="btn-group btn-group-yesno"
            layout="joomla.form.field.radio.switcher"
            >
            <option value="0">JNO</option>
            <option value="1">JYES</option>
        </field>

        <field
            name="event_place" 
            type="radio"
            label="COM_ICAGENDA_EXPORT_EVENT_PLACE_LABEL"
            default="1"
            class="btn-group btn-group-yesno" 
            layout="joomla.form.field.radio.switcher"
            >
            <option value="0">JNO</option>
            <option value="1">JYES</option>
        </field>

Changes in the file administrator/language/de-DE/de-DE.com_icagenda.ini
Add in the section „Registrations Export“ (approximately line 881):
COM_ICAGENDA_EXPORT_EVENT_PLACE_LABEL="Veranstaltungsort"
COM_ICAGENDA_EXPORT_EVENT_NEXT_LABEL="Veranstaltungsdatum"

Changes in the file src/Controller/RegistrationsController.php:
Add:
            $model->setState('event_place', $form['event_place']);
            $model->setState('event_next', $form['event_next']);
after „$model->setState('event_title', $form['event_title']);“ (approximately line 104)

Changes in the file src/Model/RegistrationsModel.php:
SQL statement extended (approximately line 168):
$query->select('e.title AS event, e.place AS place, e.next AS next,…

Added after if ($this->getState('event_title')) { (approximately line 808):

            if($this->getState('event_place')){
                $this->content .= $separator . '"' . str_replace('"', '""', Text::_('COM_ICAGENDA_EXPORT_EVENT_PLACE_LABEL')) . '"';
            }

            if($this->getState('event_next')){
                $this->content .= $separator . '"' . str_replace('"', '""', Text::_('COM_ICAGENDA_EXPORT_EVENT_NEXT_LABEL')) . '"';
            }

Added after if ($this->getState('event_title')) { (approximately line 887):

                if ($this->getState('event_place')) {
                    $this->content .= $separator . '"' . str_replace('"', '""', $item->place) . '"';
                }

                if ($this->getState('event_next')) {
                    $this->content .= $separator . '"' . str_replace('"', '""', $item->next) . '"';
                }




I hope I haven’t forgotten anything…