Convergence suggests specific import formats for Microsoft Outllok and Mozilla Thunderbird. If your exiting address book is in Outlook, export it in the CSV (Comma Separated Values) format and import it into Convergence. If the existing address book is in Mozilla Thunderbird, you should use an ldif (Lightweight Directory Interchange Format) to export and import. If you want to export and import in Convergence itself, use the Vcard 3 format.
Also, if your existing address book in Mozilla Firefox or Microsoft Outlook contains address 2 fields, after importing, that information will be lost in Convergence.