Apparently, Xanga now generates full RSS feeds for its users (none of those fugly partial RSS feeds that LJ generates by default... although LJ paid users have a workaround by using styles. My full feed is located at http://www.livejournal.com/customview.cgi?username=insomnia&styleid=90448 for instance, and if you're a paid user, you could simply switch the name in the username field.)
If you know someone on Xanga that you want to add to your friends list, you can go to http://www.livejournal.com/syn/ scroll down to the bottom, and 'Add Other Feed', using a unique account name ('xanga_username' works...) and their Xanga RSS Feed URL, in the following format:
... where the username equals their Xanga username, of course.
Yes, I know -- more RSS babble. A raising of hands for those I helplessly confused...