How to use php_value at place of php_admin_value?

[ispconfig_question]

I'm using ISPconfig 3 for webserver management. I noticed the PHP scripts are unable to set session.save_path and several other ini values. After a little searching I found out it is caused by ISPconfig settings in the vhost Apache configuration file. ISPconfig sets different session.save_path for each website. Up to this point, everything is OK. But ISPconfig uses Apache directive php_admin_value to set the PHP config value, which disallows further changes in .htaccess or runtime. I don't want to limit the scripts in setting their own session.save_path, so settings this by php_value directive (at place of php_admin_value) would be perfect for me.

Is there any way how to set ISPconfig to use php_value at place of php_admin_value for certain ini settings? If not, what can I expect if I change the .vhost Apache config file generated by ISPconfig? Will it break ISPconfig's ability to set up this website? Or will it get overwritten each time I change anything in ISPconfig for this website?