[Taz-in-Space]

Yep, there is not Scenario Maker yet.

however a workaround does exist. I got this a while ago when I asked about it.

Below written by dogscoff:

It is possible to make scenarios:
1- create your map. Most people prefer to edit a generated map than build one from scratch.
2- Decide what empires you want in the game. You may need to add extra "uber- empires". More on them later.
3- start the game with all players under human control.
4- Play the game, guiding each empire towards the situation you want it to start in (your scenario start point). Research any necessary techs, develop any necessary colonies, make any necessary diplomatic relationships, build any necessary ships.
Remember, you can expand each empire as much as you like if it helps you to reach your scenario start point more quickly, because you can always shrink it again afterwards if you want to: You can abandon planets, scrap ships/ facilities, close and re-open warp points (to get rid of diplomatic contacts), but the only thing that can't be undone is research: Once an empire has knowledge of a tech, it will always have that knowledge, so if a race is supposed to start the scenario with basic tech, you must make sure it never gets any knowledge above that level.
5- Number 4 is a long process, but it can be sped up by using the game's cheat codes or by having the different races work together (sharing research, gifting items and resources back and forth.) Uber- empires can also speed things up.
6- You might also need to create an "uber- empire" or two:
An uber- empire can be used to shape your scenario: It can do any necessary stellar manipulation work, colonise out any unwanted ruins, build any super-ships or super-colonies to be gifted to lower tech races, and leave minefields and rogue / derelict ships lying around. You can't usually have a normal race do these things because (A) it would take them ages to research the necessary techs and (B) once they have those techs, they can't get rid of them, and you might want them low-tech at your scenario start point.
To create an uber- empire, mod the game files before game start so that there is a racial tech tree with massive research, supply and construction bonuses. Create a race with this tech tree. You might also want to give it the same shipset as one of the other races, so that any gifted ships will look "native".
To create a derelict ship, just have another empire take out the derelict's enigines in tactical mode, using an engine destroying weapon. Make sure the derelict doesn't have a functioanl repair component.
When you're finished with your uber- empire (ie just before your scenario's start point), scrap/ gift all of it's unwanted ships and abandon/ gift all planets. Eliminate any unwanted uber-population on other planets by loading them into transports and hitting "eject cargo".
The uber- empire will effectively cease to exist. Ships and units will continue to function but (as long as it has no colonising or ship capture capabilities on it's rogue ships) it will never be able to have any colonies and therefore will be unable to make diplomatic contact with anyone.
7- when you have everything in the right place, run a few non-event turns to clear the message log, then send out any diplomatic Messages to explain the scenario to the human player(s).
8- Put any AI players under AI control.
Hit "end turn", "save" and distribute the savegame file as a scenario.
Easy huh? There are plenty more hints and tricks, but that's the basics of it.

If you want to get fancy, look at the tutorial scenario, and make your own Version of those files.
The specialized scenario files only work when the scenario is loaded from the scenario button, not the load game button, so it will only work well for singleplayer or hotseat games that are played using sequential turns.