Any ideas?

[ZiNgA BuRgA]

Well, there's two ways of going about it.

The dumb and silly way is just to directly fetch the feed and output it directly to the postbit.  My guess is that most people would choose this, if asked.  It's dumb and silly because results are uncached, but otherwise fine if you don't mind your server getting killed by potentially going out and fetching 10 different feeds every time someone tries to load showthread.

The "better" way is to use some cache and retrieve feeds periodically, but this is by no means something I'd consider particularly "easy".  All in all, I'd just stick a link to the external website - displaying it inline really doesn't offer that much benefit, especially considering all the effort required to make it work *well*.