I've just checked the desktop version of claws-mail, and the address book has a menu bar from which you can choose to import a vCard. Unfortunately, this menu bar doesn't exist in maemo version.
As a solution, you can install claws-email on your computer (there is linux and win versions available), import all the addresses you need and then just copy all the files from the folder addrbook to your phone, it should be in:
/home/user/.claws-mail/addrbook/