You should be aware of the Bouncycastle C# library. There are in particular two very useful classes: Org.BouncyCastle.OpenSsl.PemReader which will convert from the openssl style key you have to a bouncycastle key object, and Org.BouncyCastle.Security.DotNetUtilities, which will convert a bouncycastle key to a .NET RSAParameters object.
Here is a tiny bit of untested code that shows how to use it
using System;using System.IO;using System.Security.Cryptography;using Org.BouncyCastle.OpenSsl;using Org.BouncyCastle.Crypto;using Org.BouncyCastle.Security;using Org.BouncyCastle.Crypto.Parameters;namespace RSAOpensslToDotNet{ class Program { static void Main(string[] args) { StreamReader sr = new StreamReader("../../privatekey.pem"); PemReader pr = new PemReader(sr); AsymmetricCipherKeyPair KeyPair = (AsymmetricCipherKeyPair)pr.ReadObject(); RSAParameters rsa = DotNetUtilities.ToRSAParameters((RsaPrivateCrtKeyParameters)KeyPair.Private); } }}