Adding more than 100 feeds manually into Outlook 2007 wasn't an option and RssReader does import OPML, but doesn't export to it. So I did a little Googling and found a nice script, with an accompanying XSLT transform file. It will take RssReaders XML based export file as an input and create an OPML compliant XML file. The only thing missing here is Groups, so I had to manually regroup all my feeds :-
You can find the script here
Btw. Outlook 2007 RSS implementation in Beta1 TR is way better that the last version I've tested - but there is still room for enhancement (Remember its still beta, so it will probably change). The concept of groups is badly implemented (Especially when adding new feeds), the formatting of the posts is nothing compared to RssReaders and there's missing a "View all Unread Posts" and a "Mark all Unread Posts as Read" (It can be accomplished under Unread Mail - but I personally don't like to mix these in the same folder).