Just add it in server wide php.ini. If you have domain with CGI/FastCGI mode of PHP some of values will be taken from /var/www/vhosts/DOMAINNAME/conf/php.ini file despite of settings defined in Plesk interface. Thus priority of receiving settings for custom php.ini is following:
server-wide php.ini < PHP setting defined for the domain in the Plesk GUI < domain's conf/php.ini