did you check on that : http://www.sic.ch/de/dl_ibantool_fi_swh.pdf
you can found this on page 41
Anwendungsbeispiel: Aufruf aus C#:
[DllImport("IBANKernel.dll")]
static extern int IT_IBANConvert( String sKonto,
String sBCPC,
StringBuilder lpszIBAN,
int nIBANLen,
StringBuilder lpszBC,
int...