Click "Apply ", the extension added disappeared
You can't add .Rar to the list of Zip extensions.
.Rar files are not .Zip files.
You can turn .Rar support on and off in the Archive and VFS Plugins page. It's on by default and you don't normally need to do anything for Opus to handle .Rar files.
The config dialog explicitly stops you from doing this as a few people have had the same idea for some reason, and if you actually did it, you would no longer be able to Open .Rar files as Opus would try to interpret them as .Zip files, which they are not.