Rad Reader accepts imports (and automatically produces its own exports) using the OPML format that's commonly used in the RSS ecosystem. There are instructions on importing here https://cblgh.org/rad-reader-manual/#importing-feeds but the gist of how it currently works is that you provide the path in Rad Reader's input field to the OPML file you are trying to import. You could try to rename the XML file you have with .opml, provide the file path and see if it works :)
Basically everything that's in the purchased version is in the demo except persisting to disk. I just double-checked and importing by OPML works in the demo!
If you have any more details or can provide a subset of the XML you are trying to import, that could give me something more substantial to go on. As an aside (mostly for myself:) while trying things out right now I noticed one instance of OPML file that isn't being handled correctly right now.