Thank you for your feedback!
Pull Members to Base Type
is the refactoring that can add members to an existing interface. Just navigate to the added method and execute Pull Members to Base Type. If the base type (including interfaces) is only one, the member will be added to it. If there is more than one, a dialog will prompt you to which type you wan to pull.
If you have any other questions, please don't hesitate to write back!
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