how to add a new podcast episode to an existing itunes account

Add the new episode to the feed in the same manner in which you added the original episode, then publish (upload) the feed to your server. That's all - you don't have to re-submit it (indeed you can't). Subscribers will see the new episode pretty well immediately but the Store usually takes 1-2 days to catch up (so don't panic if it doesn't appear there immediately). Check it's OK by subscribing in iTunes.