How to include one xml file in another?

Hi,
I am pretty new to XML and trying to learn more. I am trying to figure
out if something is possible, and if so how to do it...

I created a bash script on my server to create an xml / rss file of
podcasts as they are updated. What I wanted to do was have two files
which are merged such that

1. podcasts.xml contains ONLY <item>s of podcasts. This was I just
just append new podcasts to the end of this file.

2. feed.rss.xml contains everything else and is the external link.

(I had been thinking in PHP terms perhaps when I thought this out.)
After a bunch of searching and reading tutorials I THOUGHT that using
an <ENTITY> would do the trick for this... but no luck. Can anyone
tell me if it is possible to include / call / reference  another file
from within an xml file... and if so how I am supposed to do it?

Thanks,

Craig Hagerman