Option Explicit
 
Dim strNumber As String
 
Private Sub Form_Load()
 
  strNumber = "12345678902547854785478775456554545464654654654654654746498"
  Call AddToNumber("123456789025478547854787754565545454465445646546498")
 
End Sub
 
Private Function AddToNumber(ByVal strNumber2Add As String)
 
  Dim intNumber As Integer         'akuelle Zahl
  Dim intNumber2Add As Integer
  Dim intCarry As Integer         'Übertrag
  Dim iItem As Integer            'Aktuelles Element
  Dim bReady As Boolean
  Dim strLeft As String
 
  While (bReady = False)
 
    'aktuelle Nummer für Berechnung
    If Len(strNumber) > iItem Then
      intNumber = CInt(Left$(Right$(strNumber, iItem + 1), 1))
    Else
      intNumber = 0
    End If
 
    'akuelle Zahl für Berechnung aus zu addierender Zahl
    If Len(strNumber2Add) > iItem Then
      intNumber2Add = CInt(Left$(Right$(strNumber2Add, iItem + 1), 1))
    Else
      intNumber2Add = 0
    End If
 
    'Prüfen ob fertig
    If (intNumber2Add = 0 And intNumber = 0) And (Len(strNumber) <= iItem And _
      Len(strNumber2Add) <= iItem) And intCarry = 0 Then
      bReady = True
    Else
 
      'Aktuelle Zahl + aktuell zu addierende Zahl + Übertrag
      intCarry = intNumber + intNumber2Add + intCarry
 
      If Len(strNumber) > iItem Then
        strLeft = Left$(strNumber, Len(strNumber) - (iItem + 1))
      Else
        strLeft = ""
      End If
 
      strNumber = strLeft & CStr(intCarry Mod 10) & Right$(strNumber, iItem)
 
      'Übertag berechnen
      intCarry = (intCarry - (intCarry Mod 10)) / 10
 
      iItem = iItem + 1
    End If
  Wend
 
End Function

Option Explicit
 
Dim strNumber As String
 
Private Sub Form_Load()
 
  Dim arr_vTest() As Variant
  arr_vTest = Array("a", "b", "c")
  Call DoIt(arr_vTest, 0, "")
 
  Debug.Print strNumber
 
End Sub
 
Private Function DoIt(arr_vItems() As Variant, iItem As Integer, strString As _
  String) As String
  Dim iIndex As Integer
  Dim sString As String
 
  For iIndex = LBound(arr_vItems) To UBound(arr_vItems)
    sString = strString & arr_vItems(iIndex)
 
    If iItem > 4 Then
      DoEvents
    End If
 
    If UBound(arr_vItems) = iItem Then
      'letztes element erreicht
      Debug.Print sString
      Call AddToNumber("1")
    Else
      Call DoIt(arr_vItems, iItem + 1, sString)
      Debug.Print sString
      Call AddToNumber("1")
    End If
  Next iIndex
 
End Function
 
 
Private Function AddToNumber(ByVal strNumber2Add As String)
 
  Dim intNumber As Integer         'akuelle Zahl
  Dim intNumber2Add As Integer
  Dim intCarry As Integer         'Übertrag
  Dim iItem As Integer            'Aktuelles Element
  Dim bReady As Boolean
  Dim strLeft As String
 
  While (bReady = False)
 
    'aktuelle Nummer für Berechnung
    If Len(strNumber) > iItem Then
      intNumber = CInt(Left$(Right$(strNumber, iItem + 1), 1))
    Else
      intNumber = 0
    End If
 
    'akuelle Zahl für Berechnung aus zu addierender Zahl
    If Len(strNumber2Add) > iItem Then
      intNumber2Add = CInt(Left$(Right$(strNumber2Add, iItem + 1), 1))
    Else
      intNumber2Add = 0
    End If
 
    'Prüfen ob fertig
    If (intNumber2Add = 0 And intNumber = 0) And (Len(strNumber) <= iItem And _
      Len(strNumber2Add) <= iItem) And intCarry = 0 Then
      bReady = True
    Else
 
      'Aktuelle Zahl + aktuell zu addierende Zahl + Übertrag
      intCarry = intNumber + intNumber2Add + intCarry
 
      If Len(strNumber) > iItem Then
        strLeft = Left$(strNumber, Len(strNumber) - (iItem + 1))
      Else
        strLeft = ""
      End If
 
      strNumber = strLeft & CStr(intCarry Mod 10) & Right$(strNumber, iItem)
 
      'Übertag berechnen
      intCarry = (intCarry - (intCarry Mod 10)) / 10
 
      iItem = iItem + 1
    End If
  Wend
 
End Function