No Dark Faction in Creative? I beg to differ!

For now you can start a survival game and use the console "changemode" command (You can also go back and forth with that.

But the underlying question was if we can add a "DLC Creative" scenario so no one needs to use that workaround - correct?

What you can possibly do is to copy all the .epb files from the Dark Faction scenario > Prefabs into the Creative scenario

\Empyrion - Galactic Survival\Content\Scenarios\Rise of the Dark Faction\Prefabs

to

\Empyrion - Galactic Survival\Content\Scenarios\Default Creative\Prefabs

Then they should be available in the Blueprint libary to spawn. (In case that is what you wanted to do)