roc.config.js

module.exports = {
    settings: {
        runtime: { /* ... */ },
        build: { /* ... */ },
    },
    webpack: (target) => {
        const settings = require('roc').getSettings();

        console.log(target); // web or node

        console.log(settings); // roc settings

        const config = {
            plugins: [
                // plugin one,
                // plugin two
            ]
        };

        // This config will be merged with existing config
        return config;
    }
};

// Or keep it simple: 
module.exports = {
    settings: {
        runtime: { /* ... */ },
        build: { /* ... */ },
    },
    webpack: {
        plugins: [
            // plugin one,
            // plugin two
        ]
    }
};

The function defined for the Webpack key should return an object that will be merged with the existing configuration.