I saw that it is possible: I followed the instructions of this article, an it works well : https://creativeme1807.wordpress.com/2015/10/06/steps-to-replace-null-values-in-an-obiee-pivot-table-with-no-data-or-na-or-no-description-using-custom-data-format/

if the field is a number I can not apply the char() function.

Thank you