Using cobwebch/external_import and after extendig tx_news_domain_model_news with custom fields I try to map on import those. But the new created fields are not getting recognized by mapper.

my config which relays in TCA/Overrides/tx_news_domain_model_news.php and looks as it follows

$GLOBALS['TCA']['tx_news_domain_model_news']['columns']['link_external']['external'] = [ 'news' => [ 'field' => 'tx_newsrundbrief_ext_link' ], ];

I'm using version 5.1

What I'm missing in this case?