With all their genetic changes, the Outcasts are practically aliens. Isn't that enough? :P
Though random aliens could be interesting. I think they'd live in the dark matters clouds and, in fact, be the true reason for them to have been delayed showing up or whatnot. Add a dark matter cloud to Yukon, through a small planet in it, the Junyo looking kind, and have them just be a strange species that lives there as a curiosity, much like Gammu and Primus. For the patrols, use some of those weird ships.
As an interesting idea, don't make them show up on the planet (as they live in the watery part) but have it full of researchers and robots (as they are used to dark matter, they need special robotic suits). Then you could say it was discovered by Quintaine from SP (or another Dr Quintaine) and tag them Quintaine's Men.