Convert Certificate and Private Key to .PFX programmatically in C#

Question

I have a .cer file output from a successful LetsEncrypt certificate request.

I have the original Private Key used to create the Certificate Signing Request (CSR) for

Answer 1

CryptoGuy's answer was really helpful and pointed us in the right direction.

We were still struggling to import a Binary DER file but this code fixed that:

var oc = OpenSSL.X509.X509Certificate.FromDER(bio); 

These pages were useful:

https://github.com/openssl-net/openssl-net/blob/master/ManagedOpenSsl/X509/X509Certificate.cs

https://msdn.microsoft.com/en-us/library/system.security.cryptography.x509certificates.x509certificate2.rawdata

Thanks all for your help :)