I've just looked at the db structure for wp and serendipty and it shouldnt be too hard to convert your old comments over to wp. Mostly what your doing is just changing the attribute names from one to the other. By far the easiest thing to do would be to use phpmyadmin to export your serendipity comments in csv format.
Open it in powerpoint, reorder the columns to match the order of wps comments. (of course insert/delete to get the data to fit). Save it as csv.
then import the data into the wp comments table.

Should only take 20 min of work.