using Aspose.Email;
using Aspose.Email.Storage.Mbox;

string mboxPath = "path/to/existing.mbox";
string emlFolderPath = "path/to/eml/folder";

using (var writer = new MboxrdStorageWriter(mboxPath, false))
{
    // Get all the EML files in the folder
    var emlFiles = Directory.GetFiles(emlFolderPath, "*.eml");

    foreach (string emlFile in emlFiles)
    {
        // Load the EML file
        var message = MailMessage.Load(emlFile);

        // Save the message to the mbox file
        writer.WriteMessage(message);
    }
}