Not much good news I'm afraid.
You must be able to run scripts in order to run JSMFeed.pl or JSMFile.pl. You may be able to run this if you have the ability to run scripts but not schedule them. What you'd want to do is create a simple command file (or batch file if your servers are Windows) that runs JSMFile/Feed.pl with the correct command line parameters (I'd keep JSMFile/Feed.pl completely off-world in this case for security reasons). In this case you would then have to trigger the wrapper-script by clicking on it from within a browser on any machine. For security reasons you'd want to keep the page with this link hidden, or better yet, run this link from your browser's favorites/bookmarks facility.
If that doesn't work, your only other work-around will be to get someone else to run it for you and use the JSMsg file they produce. You should first ask the source of the RSS feed, since this will allow them to share their feed with you AND many other webmasters like you. If that doesn't work, you may want to find other websites with similar interests and get them to run the feed.
I'm sorry I can't give you better options. Without the ability to schedule scripts, you are at the mercy of others to make their feeds available. In many ways it is web-masters like you who provided the impetus for producing JSMsg in the first place. But feed sources have to provide a JSMsg formatted feed in order for you to display their feeds.
Some day, the hope is, that somebody in the IETF will find a way to allow "including" XML data files within schemas. Right now, you can only include schemas in data, which is the wrong way around for it to be useful in situations like yours.