Espero te sirva éste ejemplo:
Cita: myStr = "Agregar línea al final del archivo"
Dim wtStream As New FileStream("C:\temp\test.txt", FileMode.Append)
Dim wtWrite As New StreamWriter(wtStream, System.Text.Encoding.ASCII, 1000)
wtWrite.AutoFlush = True
wtWrite.WriteLine(vbCrLf)
wtWrite.WriteLine(myStr)
wtWrite.Close()
wtStream.Close()
Suerte
