Are you familiar with the feature to import from an XML data source, and are you comfortable working with XSLT? If so, you could script an XML import and write an XSLT to convert the RSS to FileMaker's XML schema during the import.
If you're not so comfortable with XSLT, are you using FileMaker 12? FileMaker 12's "Insert from URL" script step could import the RSS into a field, which you could then parse out.
To create the effect of activating automatically, you'll have to choose a reasonable polling interval, maybe every 5 or 10 minutes, to re-trigger the script. You then schedule a server-side script or an OnTimer trigger to re-download and parse the RSS periodically.