I am kind of new myself but saw your question not being answered...
As you probably know, you can mouse-hover the default RSS feed in the admin panel, and find a "configure" button. Then put in any RSS feed.
Other than that, I don't know why you'd want to show someone's page in your admin panel. Might be better to find a relevant RSS or make your own sub-site with your own RSS with relevant topics. But if you insist here are my suggestions...
1. Search for "widget" and "admin" plugins. (But beware of plugins with few downloads or low ratings.)
2. Or of course, the admin dashboard has RSS widgets which you can configure for any RSS feed. But if you know more than I do, maybe you can change this to a 'text' widget. Then you can code an <i-frame> and put anything there.
3. Or otherwise, if you control the page you want to show, you can make it a Post, with multisite you create a sub-site just for that Post, then do RSS feed for that Post, with the RSS feed set to show the entire content of the Post.
This might show more than you want to show. But if it is your own sub-site RSS, you can enable a good free Theme such as Weaver II that enables custom CSS for that site. (Latest download at WeaverTheme.com, not yet available at WordPress.org.) Do a display:none on anything you don't want to show.
4. Or if you do not control the page you want to show (offsite)--make it show in an i-frame within a Post that you do control. Make a sub-site dedicted to this Post, as above.
5. Or... I am new to RSS. Sometimes it seems you can put in any URL and it shows something. So I don't know, but you might be able to create a static webpage which is i-framed to the page you want. After tweaking the CSS, maybe putting this static page URL as RSS feed might end up better than showing your actual RSS feed. I don't know, but that's what I'd try next.
These suggestions might not be kosher. More like something by Mr. Bean. But until you come across something better, might work.
Basically though, I don't know why you'd want to show someone's page in your admin panel. My opinion, better to find a relevant RSS or make your own sub-site RSS with relevant topics.