I am trying to import an existing Azure App Service plan (I import the existing Resource Group without any problem) and the code I use looks like this:

var appServicePlan = new Plan("ExistingPlanName",
new PlanArgs
{
    Name = "ExistingPlanName",
    ResourceGroupName = resourceGroup.Name,
    Location = resourceGroup.Location,
    Sku = new PlanSkuArgs
    {
        Tier = "Basic",
        Size = "B1",
        Capacity = 1
    },
    Kind = "linux",

},
new CustomResourceOptions
{
    ImportId = "/subscriptions/782a82df-843b-423f-b7ab-XXXXXXXXX/resourceGroups/XXXXXXXX/providers/Microsoft.Web/serverFarms/ExistingPlanName"
});