What a nightmare it is! There are no specific tools for this job. What you have to do is export all of the entries in RSS format (XML), and then import them into WordPress. There are two main problems with this; the first is that all html formating has to be redone which means editing each post; and secondly all of the URLs lose their numbers for the individual blog pages.
Fortunately I didn’t have that many blogs to convert over – just 60 or so. To convert, I imported the RSS feed from Serendipity into a copy of WordPress that was running on my local PC, tweaked the content so that any html worked properly, then I exported all of the blogs from my local WordPress and imported them into the live blog….. and voila!