You can create a custom server. See https://nextjs.org/docs/advanced-features/custom-server

Here is an example:

const express = require("express");
const next = require("next");

const dev = process.env.NODE_ENV !== "production";
const app = next({ dev });
const handle = app.getRequestHandler();
const server = express();

app.prepare().then(() => {
    server.get("/some-random-path", (req, res) => res.send("Hello, world!"));
    server.get("*", (req, res) => handle(req, res));
    server.listen(3000, "0.0.0.0", () => {
        console.log("Application started on http://localhost:3000");
    });
});

Then just run your new server file

you can merge the code from my answer about socket.io

https://stackoverflow.com/a/62547135/2068876

with the example given on Github:

https://github.com/websockets/ws#multiple-servers-sharing-a-single-https-server

try something like this (not tested, but it seems valid since the principle is the same):

./pages/api/wss1.js

import { WebSocketServer } from "ws";

const wss1Handler = (req, res) => {
  if (!res.socket.server.wss1) {
    
    console.log("*First use, starting wss1");
    const wss1 = new WebSocketServer({ noServer: true });
    res.socket.server.on("upgrade", function upgrade(request, socket, head) {
      wss1.handleUpgrade(request, socket, head, function done(ws) {
        wss1.emit('connection', ws, request);
      });
    });
    res.socket.server.wss1 = wss1;
  } else {
    console.log("wss1 already running");
  }
  res.end();
}

export const config = {
  api: {
    bodyParser: false
  }
}

export default wss1Handler;