Merge old podcast RSS feed with new one from WP

I’m creating a new WP based site for a friend who has a podcast that has 100+ episodes. I would like to make it possible for him to simply post a new podcast on the WP site and have it incorporate the new episode in the RSS feed for iTunes.

I have the podcast.rss file for the current podcast feed; aside from changing the pointers to the current location for the files on the new server (and letting iTunes know about this), how do I make sure that the current feed will have new episodes added to it, rather than creating a totally new podcast feed?

Thanks!