Open foo For Append As #bar

For i = LBound(FileArray) To UBound(FileArray)
    Debug.Print ("Processing: " & FileArray(i))
    Open (InputDirPath & FileArray(i)) For Append As #FileNumber
    Print #FileNumber, TextString
    Close #FileNumber
    Debug.Print ("Appended to " & OutputFileName & ": " & FileArray(i))
Next i

 FileToWriteTo.OpenAsTextStream(ForWriting).Write(FileToRead.OpenAsTextStream().ReadAll)

FileToWriteTo.OpenAsTextStream(ForAppending).WriteLine(FileToRead.OpenAsTextStream(ForReading).ReadAll)

Public Sub test()
    Dim fso As New FileSystemObject

    Const path As String = "C:\Users\username\path\to\file\"

    Dim helloWorld As TextStream
    Set helloWorld = fso.CreateTextFile(Filename:=path & "helloworld.txt", OverWrite:=True, Unicode:=False)

    helloWorld.WriteLine fso.GetFile(path & "hello.txt").OpenAsTextStream(ForReading).ReadAll
    helloWorld.WriteLine fso.GetFile(path & "world.txt").OpenAsTextStream(ForReading).ReadAll

    helloWorld.Close
End Sub

Stream.WriteLine FSO.GetFile((InputDirPath & FileArray(i))).OpenAsTextStream(ForReading).ReadAll

Public Sub test()
    Dim FSO As Object
    Set FSO = CreateObject("Scripting.FileSystemObject")

    Const path As String = "C:\test\"

    Dim helloWorld As Object
    Set helloWorld = FSO.CreateTextFile(FileName:=path & "helloworld.txt", OverWrite:=True, Unicode:=False)

    helloWorld.WriteLine FSO.GetFile("C:\test\Product_ID_update.txt").OpenAsTextStream(ForReading).ReadAll 'INVALID CALL OR ARGUMENT
    helloWorld.WriteLine FSO.GetFile("C:\test\RPT-4475.txt").OpenAsTextStream(ForReading).ReadAll