The mentioned problem has the solution only via customization the related files:
The first one should have new variables like __city__ and the second should have the processing of this variable in the method protected function _getPost($sType, $aEvent, $aBrowseParams = array())
It will look like the addon after 'comments' => '',: line:
'comments' => '',
'city' => $aEvent['city_or other new field name'].
BUT in this case, you will lose the automatic update of the timeline module. And always have the backup of all changed files.