No, if we want our VM to be part of an availability set, then we need to create the VM within the set.

you cannot directly add an existing virtual machine (VM) to an existing availability set in Azure. This functionality is not supported. Availability sets are configured during VM creation, and their purpose is to distribute VMs across different fault and update domains, ensuring high availability in case of hardware failures or maintenance events.

If you need to utilize an availability set for an existing VM, you have two options:

1. Recreate the VM:

Detach all data disks from the existing VM.
Create a new VM within the desired availability set, choosing the same size and configuration as the old one.
Attach the previously detached data disks to the new VM.
Reinstall any necessary software and configurations.

2. Use Azure Migrate:

If your VM resides on-premises or in another cloud provider, consider using Azure Migrate. This service offers tools to migrate workloads to Azure while potentially leveraging features like availability sets during the migration process.

Although adding an existing VM directly to an availability set isn’t possible, these alternative methods provide viable solutions to achieve your desired level of high availability. Remember to carefully evaluate your individual needs and choose the approach that best suits your specific scenario.