Hi,
I have already done this a couple of weeks ago. I have used this script for the purpose:
SCOM 2012: Bulk Override Creation for Monitors and Rules using PowerShell
You just need to specify the respective proerty that needs overriding, liek for example:
$Override.Property = 'GenerateAlert'
What you can also check is the Override Creator Tool as it has some bulk override capabilities. From Kevin Holmans blog:
You just need to check if you can override a single Param or whether the tools is only capable of disabling workflows in bulk.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Stoyan Chalakov