I understand that the tech tree for warp drives is given by event, and this is dependent on exploring some ruins?
In a custom made map (with specified locations) would I just add some ruins to a moon or something? Is there any way to make sure that the ruins allow this technology instead of, say, giving a tech or cash bonus or whatever?
Or do I have completely the wrong end of the stick?
Yes, you should add a ruin and specify that when it is explored that a tech is given. You could choose the pre war technology in a list. I have played a little with this before finding the good combination but you could either choose to have the tech given or unlock the research for the tech like in the native Shadow game.