Import RSS into iCal
Generally speaking Calendar supports CalDAV and Exchange calendars. I am not certain what an "rss-calendar" is but if it can be converted to or supports CalDAV or Exchange calendar formats you are golden.

I have not had to import a or subscribe to a calendar in quite a while so Calendar in the last couple of versions of Mac O/S may have some new supported formats. You may want to try just pointing Calendar to the URL of your "rss-calendar" and see what happens.

The other possibility is if you can export an ICAL compatible file that can then be imported into Calendar. It's not automatic syncing but it may be better than nothing.