Hi Cinthia,
No, it is not possible to add an existing VM to an availability set since you can only do that when it is first created. There is a sample script that shows how to delete/recreate a VM to make it part of availability set.
Change the availability set for a VM by using Azure PowerShell
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/change-availability-set
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Thanks.
-TP