Migrating mail from KMail is a bit of a problem, since it no longer is able to either use the mbox format, nor can it export to it. A cross-platform choice such as Opera or Thunderbird, however, won't import maildir folders.
So, what you need is a little perl script from here - just download the updated version from the link on the page, edit the lines that describe the source directory: eg
One side benefit of switching to mbox format is that you won't waste so much drive space. On a drive with 4k file sector sizes, the average "slop" lost per file is 2k, moving 20,000 messages to 20 files should save, on average, almost 50 megs (plus it's a lot quicker to open 20 files than it is to open 20,000 files).