ConvertCert.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

ConvertCertificate.ps1

using namespace System.Security.Cryptography
using namespace System.Security.Cryptography.X509Certificates

param (  
  [Parameter(Mandatory=$True)] [Alias("in")] [string]$inputFile,
  [Parameter(Mandatory=$True)] [Alias("pass")] [string]$password,
  [Parameter(Mandatory=$False)] [Alias("out")] [string]$output
)

function ConvertToPem([string]$filename, [string] $password) 
{
    try {
        $cert = [X509Certificate2]::new($filename, $password, [X509KeyStorageFlags]::PersistKeySet + [X509KeyStorageFlags]::Exportable)
        $certPem = [string]::new([PemEncoding]::Write("CERTIFICATE", $cert.RawData))

        $certAlgorithm = 
            [RSACertificateExtensions]::GetRSAPrivateKey($cert) ??
            [ECDsaCertificateExtensions]::GetECDsaPrivateKey($cert) ??
            [DSACertificateExtensions]::GetDSAPrivateKey($cert) ??
            $cert.GetECDiffieHellmanPrivateKey()              

        if ($null -eq $certAlgorithm) {
            Write-Error "Unknown certificate algorithm"
            Exit 10
        }
        
        $keyPem = [string]::new([PemEncoding]::Write("PRIVATE KEY", $certAlgorithm.ExportPkcs8PrivateKey()))
         
        return "$certPem`n$keyPem"
    }
    catch {
        Write-Error "An error occurred during certificate conversion"
        Write-Error $_
        Exit 20
    }
    finally
    {
        $cert.Dispose()
        $certAlgorithm.Dispose()
    }
}

if (-Not($output)){
    $output = [io.path]::ChangeExtension($inputFile, ".pem")
}

$pem = ConvertToPem -filename $inputFile -password $password

$pem | Out-File -FilePath $output

Program.cs

using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

var (input, password, output) = args.Length switch
{
    2 => (args[0], args[1], Path.ChangeExtension(args[0], ".pem")),
    3 => (args[0], args[1], args[2]),
    _ => throw new ArgumentException("Pass either 2 (input, password) or 3 arguments (input, password, output)", nameof(args)),
};

var pem = ConvertToPem(input, password);

File.WriteAllText(output, pem);

static string ConvertToPem(string filename, string password)
{
    using var cert = new X509Certificate2(filename, password, X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);

    var certPem = new string(PemEncoding.Write("CERTIFICATE", cert.RawData));

    using var certAlgorithm =
        cert.GetRSAPrivateKey() as AsymmetricAlgorithm ??
        cert.GetECDsaPrivateKey() as AsymmetricAlgorithm ??
        cert.GetDSAPrivateKey() as AsymmetricAlgorithm ??
        cert.GetECDiffieHellmanPrivateKey() as AsymmetricAlgorithm ??
        throw new CryptographicException("Unknown certificate algorithm");

    var keyPem = new string(PemEncoding.Write("PRIVATE KEY", certAlgorithm.ExportPkcs8PrivateKey()));
    
    return certPem + Environment.NewLine + keyPem;    
}