Just copy the contents of ~/Mail (Kde's mail folder) to ~/nsmail (Netscape's folder)
This assumes that you don't have any mail in the Netscape folder at the moment.
If that's not the case,
rename the kde files (in a separate directory of course) to something like this
.inbox.index -> .kinbox.index
.sent-mail.index -> .ksent-mail.index
...
And do the others
inbox -> kinbox
sent-mail -> ksent.mail
...
You only have to do this if you already have Netscape mail.
Backup both directories before you start.
It's not like this is a supported procedure...
I just tried it out here so I know that it works (kmail uses the same format as Netscape so there's nothing to convert)
John