Adding custom attributes to SCOM alert

Hi, 

I'm trying to setup a system where certain alerts are fed to our out of hours team via email.

I fell on the idea of using a Custom attribute on the chosen alerts and adding that CA as a criteria for notification subscription. 

If I set the custom attribute on an active alert, will that same Custom attribute stay in place for the future or does that apply specifically to that alert? 

Can the custom attribute be updated via powershell?

Need some guidance.