In fact if you use the import wizard it would import
into maildir format by
default.
I just tested it and verified it works.
If you have a mbox you can create a new mailbox in the maildir format and
copy paste the mails. also tested.
Ahh.. I just exported to MBOX which is the only option Claws offered and then
went to IMPORT and I selected the 2nd option which was MBOX / Unix format I
believe it said, and everything imported beautifully!
Chris