No.
There is no such a simple PowerShell command that will add a subnet to a classic Vnet. If you check the ARM REST API for both Microsoft/ClassicNetwork and Microsoft/Network, you will see that in Resource Manager model, "subnets" is managed both as resources under a Vnet and as a property in Vnet, while in classic model, "subnets" is just a property in Vnet.
Since "subnets" is just a property in Vnet for classic model, and "subnets" is an array of objects, you always need at least 2 requests to update "subnets". One is to get the old subnets, and the other is to put the subnets after appending the new subnets to the old subnets.
However, if you use REST API instead of pure Azure PowerShell, you don't need to export the xml file at all. Here is a script I wrote.
Add-Type -Path 'C:\Program Files\Microsoft Azure Active Directory Connect\Microsoft.IdentityModel.Clients.ActiveDirectory.dll'
$tenantName = "<you tenant name>"
$authString = "https://login.windows.net/" + $tenantName
$authenticationContext = New-Object Microsoft.IdentityModel.Clients.ActiveDirectory.AuthenticationContext ($authString, $false)
$clientId = "<the client id of your AD application>"
$key = "<the key of your AD application>"
$clientCred = New-Object Microsoft.IdentityModel.Clients.ActiveDirectory.ClientCredential ($clientId, $key)
$resource = "https://management.core.windows.net/"
$authenticationResult = $authenticationContext.AcquireToken($resource, $clientCred);
$authHeader = $authenticationResult.AccessTokenType + " " + $authenticationResult.AccessToken
$headers = @{"Authorization"=$authHeader; "Content-Type"="application/json"}
$vnet = Invoke-RestMethod -Method GET -Uri "https://management.azure.com/subscriptions/<your subscription id>/resourceGroups/Default-Networking/providers/Microsoft.ClassicNetwork/virtualNetworks/<your vnet>?api-version=2016-04-01" -Headers $headers
$newSubnet = ConvertFrom-Json '{"name": "newSubnet","addressPrefix": "10.34.0.0/29"}'
$vnet.properties.subnets += $newSubnet
$body = ConvertTo-Json $vnet -Depth 3
Invoke-RestMethod -Method PUT -Uri "https://management.azure.com/subscriptions/<your subscription id>/resourceGroups/Default-Networking/providers/Microsoft.ClassicNetwork/virtualNetworks/<your vnet>?api-version=2016-04-01" -Headers $headers -Body $body
Note: in order to use my script, you need to follow this tutorial to create a service principal.