As of 9.4.x I do not believe this is possible, have you put in a PER?
Have you looked at sending your alerts to a third party solution like "Email2DB"
Could you not duplicate your current alarm but tweak it a bit to use Field Match conditions where one of the conditions you add to the alarm is Time of Day and then you would configure the alert only to trigger outside business hours etc ?
I achieved what you want using a cloud based monitoring platform (OpsGenie to be more precise).
The main advantage I found was to effectively improve my escalation strategy by moving from business hours vs. out of business hours to:
1. Primary escalation has 5 minutes to respond
2. Primary or Secondary escalation have 5+5 minutes to response
3. Escalate to external provider via email.
It works perfectly.
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