Azure WAF Custom Rule Samples and Use Cases

Once you have implemented a WAF CustomRule, for example IP Allow, is there a way to update it with/add new IP's to the list and maintain the existing entries using PowerShell?

I see where I can create a new policy but it overwrites existing, how can I capture the current IP's and add to the list.

I see that I can go into the portal and add very easily but we are trying to automate this process.

Thanks,

Steve