Ok found the other example and now I've got a functioning add nodes function. Code below for those interested. Note - I'm pulling back an array for NewNodeID so I have to state which element I'm interested in. Your code may be different so YMMV.
function AddInterface ($NewNodeID)
{
Write-Host "Entering AddInterface function"
$TargetNodeID = $NewNodeID.GetValue(1)
$XMLInterfaceList = Invoke-SwisVerb $swis "Orion.NPM.Interfaces" "DiscoverInterfacesOnNode" $TargetNodeID
$MyInterfaceArray = @($TargetNodeID, $XMLInterfaceList.DiscoveredInterfaces, 'AddDefaultPollers')
Invoke-SwisVerb $swis "Orion.NPM.Interfaces" "AddInterfacesOnNode" $MyInterfaceArray | Out-Null
Write-Host "Exiting AddInterface function"
}