[Hippocrates]

You can "lock" the locations of a homeworld's system on a particular map, but not where within said system it will be.

When you start a new game, head on over to the map options. There's an option there for "starting locations." Click that, then start clicking on systems in the mini-map view (I'm presuming here you've already generated a map or loaded one)....as you click a particular system, it will change color to a kind of red/black symbol.

Now...whichever system you clicked first will go to the first race listed under Empires. The second system clicked will go to the race in the second "slot," and so on. You'll have to reset your starting positions each time you reload the map, I believe.

In SEIV, you could set specific starting locations using the map editor, and have them permanently associated with said map. I'm not sure if this is the case in the SEV editor, as I haven't played with it, but it might be worth checking out.