Pop and push the values of notification_classes in main.cf

[dmcmd Ndbd]

Hi,
As you know we can change the values of notification_classes in main.cf

Sample:
In terminal:
postconf -e notify_classes="delay, policy"
So it changes the values.

But in my case when user push let's say "bounce"parameter in User Interface, it means that below command;

postconf -e notify_classes="bounce"

(I've tried all others options and they're doing the same.)
So it replaces bounce. The other values (delay and policy) are missing. result is like only "bounce"

But I want "bounce" push, not to replace. I mean result I want be like "delay, policy, bounce." And also if I want to remove let's say "delay", the result I want be like "policy, bounce"
I've looked at everywhere but couldn't find any topic related to this. If you help, I'll be glad.
Best all,
Dinazor