Thank you Velocd, I've searched in the global.php and functions.php, I found this function to fetch forum's informations but found nothing concerning extra fields as we could do in vB2
I think I'll have to add an extra query to select what I want :-/
Using bitfields should be an other solution but if I do that, I'll have to use the forum.php to update informations for this field and I don' want to do that
Thanks for you answer

If someone has the solution ...