At the moment there is no Direct Restore to Azure Stack support in Backup & Replication.
I've been working with some providers who have gotten creative and worked out a way to manually restore VMs from Agent backups back into Azure Stack. They have done this with PowerShell where they import the VHD and then construct and new Azure VM using the New-AzureRmVM commandlet.
Global Technologist, Product Strategy | VMware vExpert
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