Q. If I already have an ExpressRoute circuit and wish to add additional virtual networks to it do I need to involve the carrier?
A. No. Once you have ExpressRoute provisioned with a carrier to add additional virtual networks to the circuit does not require any action on the carriers part. Instead the only required actions are to create an authorization key for each gateway connection (each virtual network will require a gateway instance) as a property of the ExpressRoute circuit and then that authorization key and the resource ID of the circuit are used to connect the gateway to the ExpressRoute circuit.
Note to use a Resource ID you need to use PowerShell or redeem the Connection first (per https://docs.microsoft.com/en-us/azure/expressroute/expressroute-howto-linkvnet-portal-resource-manager#connect-a-virtual-network-in-a-different-subscription-to-a-circuit).
Below is an example script to connect:
$id = "<ExpressRoute Resource ID>"
$authkey = '<auth key>'
$rg = "<RG Name>"
$location = "East US"
$gw = Get-AzureRmVirtualNetworkGateway -Name "<name of gateway>" -ResourceGroupName $rg
$connection = New-AzureRmVirtualNetworkGatewayConnection -Name "<connection name>" -ResourceGroupName $rg -Location $location `
-VirtualNetworkGateway1 $gw -PeerId $id -ConnectionType ExpressRoute -AuthorizationKey $authkey
