This can be done by adding:
directive into the PHP configuration file /etc/php.ini
Alternatively it is possible to specify upload_tmp_dir for one domain only. To do that add the following line into the domain's vhost.conf file:
<Directory /path/to/vhosts/domain.com/httpdocs> php_admin_value upload_tmp_dir /tmp</Directory>
Once the changes have been made run websrvmng -a -v utility for the changes to take effect.