If you want to create a customeventlog during setup, in a custom action. You can open a App.config file containing the log4net configuration and read the settings from that file at runtime.
You also can add appenders or remove appenders at runtime, but in this case I just wanted to read the configuration and create a custom eventlog:

       private string _productInstallationFolder = null;
       /// <summary>
       /// Returns C:\Program Files\MyCompany\MyProduct or C:\Program Files (x86)\MyCompany\MyProduct depending on the platform.
       /// </summary>
       public string ProductInstallationFolder
       {
           get
           {
               if (string.IsNullOrEmpty(_productInstallationFolder))
               {
                   var programFilesFolder = System.Environment.GetFolderPath(System.Environment.SpecialFolder.ProgramFiles);
                   _productInstallationFolder = Path.Combine(programFilesFolder, @"MyCompany\MyProduct");
               }
               return _productInstallationFolder;
           }
           set
           {
               _productInstallationFolder = value;
           }
       }
       public void CreateCustomEventLog()
       {
           XmlConfigurator.Configure(new FileInfo(string.Format("{0}.config", this.ProductInstallationFolder))); 
           var repository = LogManager.GetRepository() as Hierarchy;
           if (repository != null)
           {
               var appenders = repository.GetAppenders();
               if (appenders != null)
               {
                   foreach (var appender in appenders)
                   {
                       if (appender is EventLogAppender)
                       {
                           var eventLogAppender = appender as EventLogAppender;
                           EventLog.CreateEventSource(eventLogAppender.ApplicationName, eventLogAppender.LogName);
                           // Close application to allow the Windows eventlog service to refresh.
                           // When applcation is restarted the first log event will create the log file.
                       }
                   }
               }
           }
       }