Android .apk files are just zip files, is there any way to add the .apk extension to be viewed as a .zip file in the viewer pane, so that highlighting one will show it's folder structure/files there as it normally does for the other standard compression formats?
No, zip files in the viewer pane is something we get from Internet Explorer, which doesn't recognise APK files as zips.
You can add zip files to Opus's list of zip extensions so that you can open them as folders in the file display and folder tree. The ability to view zips in the viewer pane is only there because we get it for free with IE, more a bonus than an intentional feature (since it's so limited compared to opening the zip files properly in the file display).
(Someone could write a viewer plugin if they really wanted it, but it would be a lot of work just to use the viewer panel instead of either of the two available file displays to show the archive.)