[dogscoff]

Basically, you need to start a game, in control of all or most of the players. Play however many turns you need to set up all the ships, planets, politics etc, then turn some/ all empires over to the AI, send out out a few explanatory Messages and then sae it.

Distribute your savegame as a scenario.

A few things that might make things quicker:
-use the map editor.
-Consider including a special "creator" race. This race can create advance as far in tech as it likes to set up any stellar manip, create "derelict" ships and the like, and then you just abandon all their planets before game start.
-You could even mod the game to give this special race super building/ research powers and speed.
-tech and building will be much quicker if everyone is in trade agreements with everyone else. Careful not to make contact between races that are supposed to be unknown to one another though.