All the modifications I did are in the csv_export.php

1) a correction of a bug: send the correct header to the navigator (line 59)
//NPL
// header( 'Content-Type: text/plain; name=' . urlencode( $t_filename ) );
header( 'Content-type: text/csv' );
//-NPL

2) how to add the customs fields names on the first line (line 76)

foreach ( $t_columns as $t_column ) {
    if ( !$t_first_column ) {
        echo $t_sep;
    } else {
        $t_first_column = false;
    }

    if ( strpos( $t_column, 'custom_' ) === 0 ) {
        $t_column_title_function = 'print_column_title';
        helper_call_custom_function( $t_column_title_function, array( $t_column, COLUMNS_TARGET_CSV_PAGE ) );
    } else {
        $t_function = 'print_column_title_' . $t_column;
        $t_function( '', 'ASC', COLUMNS_TARGET_CSV_PAGE );
    }
}

//NPL Add of the customs fields names
$t_related_custom_field_ids = custom_field_get_linked_ids( $t_project_id );
foreach( $t_related_custom_field_ids as $t_id ) {
if ( !$t_first_column ) {
echo $t_sep;
} else {
$t_first_column = false;
}
$t_def = custom_field_get_definition( $t_id );
echo string_display( lang_get_defaulted( $t_def['name'] ) );
}
//-NPL

echo $t_nl;

3) how to add the customs fields values for each bug (at the end)
//NPL Ajout des champs personnalisés
$t_related_custom_field_ids = custom_field_get_linked_ids( $t_project_id );
foreach( $t_related_custom_field_ids as $t_id ) {
if ( !$t_first_column ) {
echo $t_sep;
} else {
$t_first_column = false;
}
$t_def = custom_field_get_definition( $t_id );
print_custom_field_value( $t_def, $t_id, $t_row['id'] );
}
//-NPL

    echo $t_nl;