Did you see this near the top of the FAQs list?:
[ul][li]Always re-use a single window instead of opening new windows -- New tabs instead of new windows. Also how to make a hotkey to show the last active window.[/li][/ul]
Note that you'll only get what you want (adding the double-clicked folder as a tab in addition to all your default tabs) if the default lister is already open. At the moment I don't think there is a (good) way to make Opus open the default lister and then add a tab to it when double-clicking a folder.
(Put another way, you can make double-clicking the folder add a tab to the last-used existing window, but if no windows are currently open then a new window will open with just that folder/tab.)