A Simple PHP Wordpress Script

This is a simple job for a coder who knows what they are doing. I have a wordpress blog that outputs an rss feed. What I would like to do is to randomly take out a post from that feed. From that feed I would like to take the first 500 characters. Then select from a text file called [login to view URL] one of the introductions and place that above the 500 charcters. (note the text files will contain introductions and be seperated by a # to identify the break) Next select from a text file called [login to view URL] one of the closes and place that below the 500 characters. Then recreate this a new feed. I need this to be done automactically for every post in the orginal feed so each of the finished intro/500 characters/exit has its own feed.