вообщем получилось добавить...если в двух словах, то примерно так.. после ComputeSignature в методе sign класса XmlSignatureManager
X++:
......
xmlDigitalSignature = signedXml.GetXml();
XmlSignatureManager::addPrefix("ds", xmlDigitalSignature);
signedXml.LoadXml(xmlDigitalSignature);
signedInfo = signedXml.get_SignedInfo();
references = signedInfo.get_References();
references.Clear();
signedXml.ComputeSignature();
recomputedSignature = System.Convert::ToBase64String(signedXml.get_SignatureValue());
XmlSignatureManager::replaceSignature(xmlDigitalSignature, recomputedSignature);
....