Kontact

1. If you store your email in Kontact as mbox files just straightforwardly import them into Apple's Mail application.

2. If you store your email as mailbox folders, the procedure is a bit more complicated. - in each mail folder to be imported run the script converting mailbox into mbox (the script doesn't support nested folders so one has to run it for each folder separately, spaces are not allowed for the script to work) - import mbox into Mail TBC...