In short, no. The only time you can add a VM to an availability set is doing its creation
Refer to https://zcusa.951200.xyz/en-us/azure/virtual-machines/windows/change-availability-set
You can add a VM to an availability set only when you're creating the VM. To change the availability set, you need to delete and then re-create the VM.
However, you can use the script in this Learn article to automate the process of recreating the VM.
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hth
Marcin