So just to clarify because I may have become confused somehow... are you wanting to make sure when you double-click on these other extensions, that Opus opens them in the lister as if they were folders?
If so, then as I mentioned before go into the Dopus file type editor (Settings->File Types...) for these extensions (System File Types->xpi) and make sure the dblclk event (on the Event tab) is set to run the Go command.
Tanis:
I've done some more testing - and like you I added XPI files to the list of extensions handled by Dopus but like Walser this did NOT allow me to double click them to enter as folders - ie. they still opened in PowerArchiver.
What seems to be happening is that if you ALREADY have some specific command or program being called for the dblclk event (in my case "C:\Program Files\PowerArchiver\POWERARC.EXE" "%1"), adding the extension of the file to Dopus 'zip list' will NOT overwrite the existing dblclk event with the required go command. I found that if I deleted the current dblclk event command - thus making it - then adding the extension to Dopus 'zip list' does indeed update the event with the go command. So I guess the 'bugette' that I was referring to is still there (I'm running v8.1.0.8 ANSI) if this can be considered a bug. Keep in mind that the Dopus dblclk event is Dopus specific and does not alter the open action for the file type which would affect the whole system - i.e. Explorer - and which MOST filetypes default to being subjected to when double-clicking on them (with the lack of an explicit dblclk event being defined).