See https://www.reddit.com/r/operabrowser/wiki/m2tips/import_contacts_from_other. You have to export the abook as a csv file and use the converter page to convert it to Opera's adr format. Only some fields are supported in the conversion.
Opera Mail can also generate address book entries from sent messages. See the "Outgoing" tab in the account's properties. For IMAP, if you sent your IMAP sent folder, it'll use all of those too.