It is possible for Paid and Permanent Account holders to add journals from certain other sites to their friends list by using LiveJournal's syndication feature. More information about this can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=137 .
Now (technically) free users can do this as well, provided the rss feed has already been created, and it is popular enough for them to be able to afford it.
I don't have time to rewrite it, so I thought I'd see if anyone else wants to rewrite it, as you all do them so beautifully ;o)