add_filter('post_grid_media_source_list', 'post_grid_media_source_list_20200614', 10);
function post_grid_media_source_list_20200614( $sources){
$sources['custom_source'] = __('Custom media source','post-grid');
return $sources;
}
How to add media source options
https://pickplugins.com/documentation/post-grid/action-hooks/media_source_options_id/