Unfortunately, what you're trying to do isn't possible.
The way RSS feeds typically work is that they'll only show the 10-most recent news items. This is why when adding a feed into Google Reader, you'll have 10 unread posts immediately available.
Most RSS readers will save content over time, which is why you can view posts further back on Google Reader but not on Outlook. The same scenario should be true if you had been using Outlook all this time rather than Google Reader. It's expected behavior.
In the end, treat RSS as a way to receive new posts. If you want to read older posts, most sites would want you to resort to looking at the actual website.
Edit: In some cases, it may be possible to export the saved news from a reader and import into another. In the case of Google Reader, you can only export the URLs to all your feeds, it will not export the posts inside those feeds.
