Assuming you know how to use SM mode:
1. Generate a new game as normal, with Sol system options.
2. Set NPR generation chance to 0.
3. Go to the System View (F9), and generate new star systems until you get one with a habitable planet.
4. Create a player empire on the new planet.
5. Delete the player empire on Earth.
6. Create an NPR on Earth.
7. Set NPR generation chance back to whatever you want it to be.
8. Optionally, create a path of jump points between the two systems, if you want to ensure a quick meeting.
And that should be it.