When you set up Gmail IMAP in Opera Mail, it'll auto-create contacts for all messages in "[Gmail]/Sent Mail".
If you want, you can turn that off. Just uncheck "add sent messages to contacts" on the Outgoing tab in the account's properties to turn this off. But, if you don't want it to happen in the first place, you create the Gmail account in Opera without entering your password. Then, after the account is created, go into its settings and turn that off. Then enter your password.
In Opera's contacts panel, it doesn't have separate view for "collected addresses. But, you can create a folder view named that and move collected addresses to it. Or, create one called Contacts for manually-added contacts.
As for importing, a former Opera employee has a converter on his site. See http://hallvord.com/opera/conversion1.htm. You export your Gmail contacts to a CSV file and then use the site to convert them. The site only supports converting over the email address and display name I think. The converter is generic, so you have to pick what field in the csv file is for the email address etc. Also, when you save the adr file the site creates for you, it might download as a php file. Just rename it to have a .adr extension. Then, in Opera, you goto "Menu -> settings -> import and export -> import contacts" and point it to the adr file.
The other option is to export Gmail Contacts as csv or vcf and find a converter that converts it to adr format.