Here are a couple of tools that will help you in the mapping:
http://www.webreference.com/perl/tools/
RSS Creator is a PERL script that will create a simple RSS file
RSS Channel Editor is a PERL script that will create a RSS channel
For each tool, click on the "Try It" link and try out your mapping. Once you figure out the mapping, then you can use a variety of languages that will parse your XML input and spit out the RSS Xml.