Re-assign a function to the exports when import as a namespace

Question

a.ts:

export const add = () => {
  return 'ok';
};

I am trying to re-assign a new add function like this:

import * as a from './a';

a.add = () => {
  return 'not ok';
};

console.log(a.add());

TSC throws an error:

Cannot assign to 'add' because it is a read-only property.ts(2540)

It works fine when I use es6. How to correct the TS type of the namespace a so that I can re-assign a new function with the same signature?

I don't want to do this: //@ts-ignore and (a as any).add = () => {}. It may be related to ts config.

tsconfig.json:

{
    "compilerOptions": {
        "target": "es6",
        "module": "commonjs",
        "lib": [
            "ES2015",
            "es2016",
            "dom",
            "es2018.promise"
        ],
        "allowJs": true,
        "jsx": "react",
        "outDir": "./build",
        "rootDir": ".",
        "noImplicitAny": false,
        "strictNullChecks": true,
        "strictFunctionTypes": true,
        "strictPropertyInitialization": true,
        "noImplicitThis": true,
        "alwaysStrict": true,
        "moduleResolution": "node",
        "allowSyntheticDefaultImports": true,
        "esModuleInterop": true,
        "experimentalDecorators": true,
        "emitDecoratorMetadata": true,
        "resolveJsonModule": true
    },
    "exclude": [
        "dist",
        "docs",
        "build",
        "node_modules"
    ]
}