angular-librarian uses the 'plugins' property to define the plugins it needs to use for testing. Normally when testing with karma, the system will find anything in the node_modules folder that starts with karma- and make it part of the testing automatically. I think I see the problem you're trying to solve though. If the developer needs a custom plugin, they must add it to this array but if it doesn't exist, it'll require them to know all the pieces ngl uses so they can add a simple plugin.
I'm not sure if you know this but you can set the plugin array to be plugins: ['karma-*'], This would restore the original Karma setup and developers only need to npm install any karma plugin to begin using. If they have a custom plugin, they can still add it to the plugins array and will be merged like normal. Does this make sense? I can elaborate if needed.