Well, it looks like you can't. The key thing to know is that jest.spyOn is just sugar for the basic jest.fn() usage. We can achieve the same goal by storing the original implementation, setting the mock implementation to to original, and re-assigning the original later:

import * as app from "./app";
import * as math from "./math";

test("calls math.add", () => {
  // store the original implementation
  const originalAdd = math.add;

  // mock add with the original implementation
  math.add = jest.fn(originalAdd);

  // spy the calls to add
  expect(app.doAdd(1, 2)).toEqual(3);
  expect(math.add).toHaveBeenCalledWith(1, 2);

  // override the implementation
  math.add.mockImplementation(() => "mock");
  expect(app.doAdd(1, 2)).toEqual("mock");
  expect(math.add).toHaveBeenCalledWith(1, 2);

  // restore the original implementation
  math.add = originalAdd;
  expect(app.doAdd(1, 2)).toEqual(3);
});

The need for this will be addressed by https://github.com/facebook/jest/issues/6180 When we can conditionally return different values from mocks.