Решилось следующим образом:
X++:
System.Security.Cryptography.AsymmetricAlgorithm alg;
alg = CertificateEncryption.get_PublicKey();
gost3410 = CryptoPro.Sharpei.Gost3410::Create();
gost3410.FromXmlString(alg.ToXmlString(false));
encryptedKey = CryptoPro.Sharpei.Xml.CPEncryptedXml::EncryptKey(sessionKey, gost3410);