I tried to do the same thing and managed to get all my present mail - not an unmanageable amount - from evolution to kmail by converting the mbox format to maildir and then doing a "select all", "copy", from each evolution dir and pasting into the kmail directories (some of which I had to create or recreate such as it is.
My problem now is that there does not seem to be a mechanism for an easily moving the many, many, etc. filters that I have created in evolution and I don't want to have to redo them all again in kmail.
Hope this helps some.
Ogre
I know you believe that you understood what you think I said, but I am not sure you realize that what you heard is not what I meant.