You'll want to enable guest customization in order to set the IP within the VM you are importing. If it's a Windows VM I wouldn't allow it to change the SID though.
Give this a shot but change the code in the second to last line to command line changes for NIC within OS depending on OS version (example for Windows = netsh interface ip set address name="Local Area Connection " static $IP 255.255.255.0 192.168.1.1). You may have to write this out to get the network interface name.
$IP = $viName.ExtensionData.GetNetworkConnectionSection().NetworkConnection[0].IpAddress
$CiVmGc = $viName.ExtensionData.Section | Where {$_.GetType() -like "*GuestCustomizationSection"}
$CiVmGc.Enabled = $true
$CiVmGc.ChangeSid = $false
$CiVmGc.CustomizationScript = [string]::join([environment]::NewLine, "Command line code to set IP address depending on OS via $IP variable")
$CiVmGc.UpdateServerData()
-Eric