The webmail facility currently supports VCARD and CSV files types for importing Contacts from other Mail applications.
Once you have exported your address book from another application, login to the webmail facility and click on Address Book in the top right.
On the next screen you will see Import in the top left hand corner. Click this and then browse to where you've saved your exported address book and click ok. All contacts will be added to the existing list, unless you tick the option to Replace the entire address book.
Note: If you can't find an answer to your problem click Here to open a support ticket (requires log in).