Solved:

-Open file plugins/export_data/admin.php
-Go to line 65 at the start of export variables (from 'SELECT')
-Add the needed variable (i.e. 'rating_score' after )one of the other variables
-Save the file

SELECT
    i.id,
    file,
    date_available,
    date_creation,
    rating_score,
    ...

To find the name of available variable (if you can't access to database) open the installation file "piwigo_structure-mysql.sql" and look into the piwigo_images table definition starting at line 181.