Dim k_lines() As String = System.IO.File.ReadAllLines("J:\Daten\konto.txt")
Dim k_fields() As String
Dim fieldindex As Integer = 8
For i As Integer = 0 To k_lines.Length - 1
    k_fields = k_lines(i).Split(";"c)
    If k_fields.Length > fieldindex AndAlso _
       String.IsNullOrEmpty(k_fields(fieldindex)) Then
       k_fields(fieldindex) = "999999999"
       k_lines(i) = String.Join(";", k_fields)
    End If
Next i
System.IO.File.WriteAllLines("J:\Daten\konto_korrigiert.txt", k_lines)