Bootstrap existing Azure ARM VM using following command:
knife bootstrap azurerm <server> --azure-resource-group-name <resource_group_name> --azure-service-location <location>
$ knife bootstrap azurerm myVMName --azure-resource-group-name myResourceGroup --azure-service-location 'westus'
You can use other options like --azure-dns-name, --bootstrap-version with this command and there are many more options which can be used with this command. Use --help to identify more options.
NOTE: This command only works for single VM bootstrap.