Can you change what the original colony is with your custom human lost colony by editing the UNE?
And are you sure there are no other Human empires?I saw that Dragatus said no, so maybe there's something else to it, but in my experience you can definitely change what the original empire is. My playthroughs usually have a force-spawned "Sacrosanct Human Republic" (with the Post-Apocalytic Origin, so you don't have to stick with Planetary Unification) controlling Earth and a Lost Colony "Technocracy of Deneb". The first contact dialogue from right after you decipher communications is a little bugged and acts like you hacked in to the other Human empire, but other than that it works fine.
And are you sure there are no other Human empires?
I guess it works if you have a Human empire, with the other Human empire force-spawned in Sol.
But in any other instance, every time I try Lost Colony by force-spawning both the colony and the originator, I get both my intended empires, but also another completely random empire.
I saw that Dragatus said no, so maybe there's something else to it, but in my experience you can definitely change what the original empire is.