Yep, squirrelmail does not have an option at this time for importing of the address book. (at least none I can find)
You may be able to do this manually, by editing the
@DOMAINNAME.abook files located at /home/CPANELUSERNAME/.sqmaildata/ (you can use cpanel file manger to access this)
This however has to be a specific format,
Anyone else got a solution, feel free to post here.