Perhaps
/// <summary>
/// Export a certificate to a PEM format string
/// </summary>
/// <param name="cert">The certificate to export</param>
/// <returns>A PEM encoded string</returns>
public static string ExportToPEM(X509Certificate cert)
{
StringBuilder builder = new StringBuilder();
builder.AppendLine("-----BEGIN CERTIFICATE-----");
builder.AppendLine(Convert.ToBase64String(cert.Export(X509ContentType.Cert), Base64FormattingOptions.InsertLineBreaks));
builder.AppendLine("-----END CERTIFICATE-----");
return builder.ToString();
}