Adding a service endpoint to an existing virtual network is pretty straightforward. All you need to do is run the following cmdlet:

Get-AzureRmVirtualNetwork -ResourceGroupName PacktPub -Name PSVNet | Set-AzureRmVirtualNetworkSubnetConfig -Name NSubnet  -AddressPrefix "192.168.1.0/24" -ServiceEndpoint "Microsoft.Storage" | Set-AzureRmVirtualNetwork