Here is what I read in one of the configuration file:
# ATTENTION!
# DO NOT MODIFY THIS FILE OR ANY PART OF IT. THIS CAN RESULT IN IMPROPER PLESK
# FUNCTIONING OR FAILURE, CAUSE DAMAGE AND LOSS OF DATA. IF YOU REQUIRE CUSTOM
# MODIFICATIONS TO BE APPLIED TO THE CONFIGURATION, PLEASE, PERFORM THEM IN THE
# FOLLOWING FILE(S):
#[PATH]/vhost.conf
#[PATH]/vhost_ssl.conf
#[SUBDOMAIN_PATH]/vhost.conf
It seems it is possible to do what I need (at least by using Apache php_admin_flag, php_admin_value, ecc...)
I don't think I can upgrade to other Plesk versions on my server.