iirc from the time when you had to install haxchi manually, ds games are just in a rom.zip file in a folder named (10 or 0010). I think it's the same for all games, so those are the easiest to work with.
.app is not unpacked, what you want is Loadiine format which Wii U USB Helper lets you extract, that contains code/content/meta folders with all the individual files and the ROM should be somewhere in there.
.app is not unpacked, what you want is Loadiine format which Wii U USB Helper lets you extract, that contains code/content/meta folders with all the individual files and the ROM should be somewhere in there.