@Gary Leow
The only way to update a policy currently with PowerShell is running Set-AzStorageAccountManagementPolicy. This will override any existing rules you had in place so you will need to include your old rules. The easiest way would be to take the current JSON of your policy and add the rules there. You can also easily add a rule or view the current JSON of your rules in the Portal.
I understand this is probably not as easy as it could be and have submitted feedback to the product team on this. I hope the information I provided was helpful. Please let us know if you have further questions or issues and we will be happy to assist.
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