declare module "koa" {
    namespace Koa {
        ...

        export interface Request {
            ...
        }
        ...
    }
    ...
    namespace Koa {}
    export = Koa;
}

/// <reference path="globals/koa/index.d.ts" />
/// <reference path="koa.d.ts" />

import koa = require("koa");
...
app.use(ctx => {
    console.log(ctx.request.body); // error: Property 'body' does not exist on type 'Request'
});

declare module "koa" {
    namespace Koa {
        export interface Request {
            body: any;
        }
    }

    export default Koa;
}

import {Request} from "koa";

declare module "koa" {
  interface Request {
    body: any;
  }
}

import * as bodyparser from 'koa-bodyparser';
...
app.use(bodyParser());

ctx.request.body