It seems nobody wants to help me :( 
I've "imported" the messages by copying tb's files to kmail's directory and when I started kmail it picked them up.

Could you send me a copy of the effected folder with the mails (maybe directly and not via bugzilla)? Without a test archive I can't reproduce the problem. Also a info about the TB version would be helpful.

Hmm I almost forgot about this, today I tried the import operation again and the problem is gone. I don't know if there is something different in kmail, kdelibs or anything else, because I've upgraded gcc last week so I've had to upgrade everything and kmail, kdelibs and everything that could affect this was upgraded too. The import works without problems now and imports everything - even the folders that crashed at the time I wrote this bug report. 
I switched to back to TB in between because I needed to write mails with inline pictures and I couldn't find it in kmail. Today when i tried importing everything back in kmail the import was ok, but kmail crashed a lot :( I'll try to disable some new flags (kdehiddenvisibility) and if it still crashes I'll try to get a backtrace for this crashes for another bug report because I really like kmail because it's so snappy and it's integrated in kde.

I face the exact same problem using thunderbird 2.0 and kmail 1.9.7.

I begin importing my folders, but at one point it kind of freeze. I have many subfolders (Drafts, Factures, Inbox, Sent, Templates) and it always freeze before reaching the "Sent" folder. So in kmail the "Drafts", "Factures" and "Inbox" are imported. I do not know if all the messages were imported though. 
 
 I straced kmailcvt. It seems to enter a kind of loop somewhere : 
 read(9, "\0\0\0\6kmail\0\0\0\0\20anonymous-12234\0\0\0"..., 46) = 46 
 unlink("/tmp/kde-nicolas/kmailcvtdE2Qic.tmp") = 0 
 read(3, 0x6402e4, 4096)                 = -1 EAGAIN (Resource temporarily unavailable) 
 select(13, [3 4 5 7 9 11 12], [], [], {0, 0}) = 0 (Timeout) 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 stat("/tmp/kde-nicolas/", {st_mode=S_IFDIR|0700, st_size=4096, ...}) = 0 
 open("/tmp/kde-nicolas/kmailcvtNChdHb.tmp", O_RDWR|O_CREAT|O_EXCL, 0600) = 14 
 umask(0)                                = 022 
 umask(022)                              = 0 
 fchmod(14, 0600)                        = 0 
 getgid()                                = 1000 
 getuid()                                = 1000 
 fchown(14, 1000, 1000)                  = 0 
 fcntl(14, F_SETFD, FD_CLOEXEC)          = 0 
 fcntl(14, F_GETFL)                      = 0x8002 (flags O_RDWR|O_LARGEFILE) 
 fstat(14, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0 
 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2af9e3892000 
 lseek(14, 0, SEEK_CUR)                  = 0 
 fstat(14, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0 
 read(14, "", 4096)                      = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 [...many exactly the same...] 
 read(13, "tie 1.ZIP\"\r\n\r\nUEsDBBQAAAAIAOCSbC"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 write(14, "From - Fri Apr 09 14:01:30 2004\r"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 read(13, "+gbXvyrYKu7j2FdNqce/qbZPCA7Fc+gU"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 write(14, "uLJ0DqtwmAqfTQlEJFpEgwKJePxyolky"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 read(13, "H2Vn+vrpJtr17QfunGpUtLJ85/H81f+7"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 write(14, "+GYpvW6ABxxW8opotY6phDJUsdO9aGCW"..., 4096) = 4096 
 [...] 
 After that it is always this patern repeated: 
 read(13, "+gbXvyrYKu7j2FdNqce/qbZPCA7Fc+gU"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 write(14, "uLJ0DqtwmAqfTQlEJFpEgwKJePxyolky"..., 4096) = 4096 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 fstat(13, {st_mode=S_IFREG|0755, st_size=29105230, ...}) = 0 
 with different values in for the read and write, always looks like garbage... 
 
 I stoped the strace when the log was about 200MB... 
 
 Could it be because the folder in thunderbird was compressed?

Is the problem present? Could you may find out where (on which file) it freezes (may search in the strace log for the last open in the directory you import from)? Could you may provide output of 'tree' from the import dir (maybe send it directly to me)?