How do I burn to a non-emtpy CDRW?

I've played around with the sample NeroCOM, and have managed to get it to collect specific files and burn them to an empty CDRW, but I cant work out how to write on top of an already burned one. Which is what I need to do

Private Sub Burn_Click()
Dim xmlFilesDir As String
Dim xmlFiles As Variant
Dim drives As INeroDrives
Dim isotrack As NeroISOTrack
Dim file As NeroFile

xmlFilesDir = Mid(App.Path, 1, InStrRev(App.Path, "\")) & "dataroot\"

xmlFiles = Array()
ReDim xmlFiles(7)
xmlFiles(0) = "cntrLOCATION.xml"
xmlFiles(1) = "cntrPROFILE.xml"
xmlFiles(2) = "cntrTAC_REC.xml"
xmlFiles(3) = "cntrCOMS.xml"
xmlFiles(4) = "cntrNHSSUPPLY.xml"
xmlFiles(5) = "cntrPS.xml"
xmlFiles(6) = "HTML-OrgChart_files\HTML-OrgChart_image001.gif"

btnAbort.Enabled = True

Burn.Enabled = False

Set Folder = New NeroFolder
Set drives = nero.GetDrives(NERO_MEDIA_CDRW)
Set drive = drives(AvailableDevices.ListIndex)
Set isotrack = New NeroISOTrack

isotrack.Name = "Completed Forms"
isotrack.RootFolder = Folder

For i = 0 To UBound(xmlFiles) - 1
If Dir(xmlFilesDir & xmlFiles(i)) <> "" Then
Set file = New NeroFile
Folder.Files.Add file
file.Name = xmlFiles(i)
file.SourceFilePath = xmlFilesDir & xmlFiles(i)
Set file = Nothing
End If

Next

Folder.Name = "dataroot"

isotrack.BurnOptions = NERO_BURN_OPTION_CREATE_ISO_FS + NERO_BURN_OPTION_USE_JOLIET

drive.BurnIsoAudioCD "NoTitle", "NoArtist", 0, isotrack, Nothing, Nothing, NERO_BURN_FLAG_SIMULATE + NERO_BURN_FLAG_WRITE, 4, NERO_MEDIA_CDRW

GoTo quit

handle_error:
strMessages = strMessages + Err.Description + Chr(13) + Chr(10) + nero.LastError
edtMessages = strMessages
quit:
End Sub