I don't use Kmail, as I use Mozilla as my mail client in both OSs. If you use Outlook in win, I think you might want to switch to Mozilla
That said, I think kmail just uses standard mbox files for mail, so hopefully you can modify this for your own use. I've heard this successful with Eudora and Kmail, but never Mozilla and Kmail...
I do exactly what you want to with Mozilla, so this might be a little helpful. I think your primary stumbling block is "what email client in Win uses the same email format as Kmail..." I don't know that answer to that. If you wanted to try Moz, here's what I do.
Partition 1 = win (ntfs)
Partition 2 = shared (fat)
Partition 3 = mandrake (reiser)
Installed Mozilla in Mandrake, let it set up its default Mozilla directory (for me its /home/user/.mozilla/). The cool thing is that the directory structure for Mozilla is consistent across both OSs, so inside you'll find a folder called "default" and inside that you'll see an 8 character folder name (I'm pretty sure its randomly generated for security reasons... no point in having a predictable location for all email storage
Close Mozilla. Copy the randomly generated folder into your shared partition; make sure the partition is fat so that both OS can read and write (there are issues about writing to NTFS from linux that I didn't care to explore
Re-open Mozilla and go to Mail and Account settings. Do into server settings, and you'll see a path for local folders. Change the path to point to your shared folder, test it.
Boot into Win. Lather-rinse-repeat.
One "catch" I encountered (stupidly) was that you need to leave Mozilla set (in both OS) to leave mail on the server. That way you won't download it in one OS and it not be available in the other. The inboxses seem to stay separate. I'm sure there's a way around this... but I couldn't be bothered.
I've been doing this is Mozilla 1.3 and never a problem.