foo.d.ts

export namespace Foo {
  export const bar: boolean;
  export const quux: (a: boolean) => string;  
}

export default Foo;

export const baz: string;

just-import.ts

import Foo from 'foo';
import { baz } from 'foo';
import Quux from 'quux';

console.log(Foo.bar);
console.log(baz === 'something');
console.log(Quux.Foo === Foo);

quux.d.ts

import FooFoo from 'foo';

export namespace Quux {
  export const Foo: typeof FooFoo;
}

export default Quux;

tsconfig.json

{
  "compilerOptions": {
    "strict": true,
    "noEmit": false,
    "baseUrl": "./",
    "typeRoots": ["./"]
  },
  "include": ["**/*"]
}