If you’re talking about subscribing to a page’s wall (not notifications feed), then yes, you can do that.
To find the feed for the wall, the easiest thing is to visit the target Facebook page in Firefox. Click the RSS icon in the address bar and choose “Subscribe to THE PAGE NAME”. This will bring you to the actual feed. Copy the URL of the feed, and set it up as a new Facebook feed in your Lifestream settings (Lifestream > Feeds > Add a Feed > Facebook). And you’re done.
The reason you can’t subscribe to a page’s notifications feed is that there is no such thing. Pages are connected to user accounts, and as such they don’t have notifications feeds of their own. If you do see a notifications feed while you’re administering a Facebook page, it’s the feed of the administering user, not of the page itself.