It was rather easy because I didn't do the hard part (thanks authors of pyrfeed!)and now uRSSus has a simple "import my subscriptions from Google Reader" action.
All I can say is "it works for me" and "Google sure strips data from the feeds!".
It will import your tags as top-level folders (merged with whatever you already had).
Should be smart enough not to duplicate feeds or folders.
If a feed is in more than one folder, it will show in the first listed by google (no idea what the order is).