...
For i As Integer = 0 To n
    Dim D As Date = DateFrom.Date.AddDays(i)
    'Abfrage ob Datum schon vorhanden ist
    If mDT.Select("Datum = '" & D & "'").Count = 0 Then 
        Dim DR As DataRow = mDT.NewRow
        ...
        mDT.Rows.Add(DR)
    End If
Next
...